Molecular cloning and characterization of the porcine calcitonin gene-related peptide receptor.

Calcitonin gene-related peptide (CGRP) receptors (CGRP-Rs) are widely distributed throughout the central and peripheral nervous systems. A novel CGRP-R was identified from a porcine lung complementary DNA library. Sequence analysis indicated that the CGRP-R is 462 amino acids in length and shares 93% sequence identity with the human CGRP-R. Northern blot analysis indicated a messenger RNA species of 5.4 kilobases, which is abundantly expressed in the lung. Ligand binding studies of the cloned CGRP-R expressed in human embryonic kidney (HEK-293) cells showed the presence of high affinity receptor for CGRP with a Kd of 38.5 pM. The pharmacological profiles of various ligands competing for [125I]CGRP binding to the expressed receptor were in accordance with those for the natural receptor. Binding of [125I]CGRP to the expressed receptor was decreased in the presence of a nonhydrolyzable analog of GTP, guanosine 5' (gamma-thio)-triphosphate. In functional studies, CGRP stimulated the activation of adenylyl cyclase with an EC50 of 2.5 nM. The linear analog of CGRP, diacetoamidomethyl cysteine CGRP, did not affect adenylyl cyclase activity on its own or in the presence of CGRP. Furthermore, the CGRP receptor antagonists, CGRP-(8-37)alpha, inhibited the CGRP-mediated response in a competitive manner. Collectively, the binding and functional data demonstrate that we have cloned a porcine CGRP type 1 receptor. The availability of the CGRP-R complementary DNA will allow us to examine its participation in pathophysiological processes.

Ser165 of transmembrane helix IV is not involved in the interaction of catecholamines with the alpha-2a-adrenoceptor.

Molecular modeling studies have predicted that the beta-hydroxyl group of the catecholamines interacts with the beta 2-adrenoceptor at the serine residue at position 165 (Ser165) located on transmembrane helix IV; however, this has not been confirmed by site-directed mutagenesis. It has been inferred that this site, which is conserved in all of the nine known alpha- and beta-adrenoceptor subtypes, is also involved in the interaction of catecholamines with the alpha 2a-adrenoceptor. To test the hypothesis that the beta-hydroxyl group of the catecholamines interacts with Ser165 of the alpha 2a-adrenoceptor, we prepared a mutant alpha 2a-adrenoceptor where Ser165 was mutated to alanine. Mutation of Ser165 of the alpha 2a-adrenoceptor to alanine had no effect on the affinity of dopamine (which lacks the beta-hydroxyl group) or either enantiomer of norepinephrine or epinephrine (both of which possess the beta-hydroxyl group), indicating that Ser165 is not involved in the interaction of the catecholamines with the alpha 2a-adrenoceptor. We have previously shown that mutation of Ser90, located in transmembrane helix II, to either alanine or cysteine produces a selective reduction in the affinity of the (-)-enantiomers of the catecholamines for the alpha 2a-adrenoceptor, with no effect on the (+)-enantiomers or the corresponding beta-desoxy analogs. This is consistent with the known stereoselectivity involved in the interactions of catecholamines with the alpha 2a-adrenoceptor. The results of the present investigation indicate that Ser165 is not involved in the interaction of catecholamines with the alpha 2a-adrenoceptor. Because all known alpha-adrenoceptor subtypes have a serine residue at a position corresponding to Ser90 of the alpha 2a-adrenoceptor, it would appear that this site represents an important point for attachment of the beta-hydroxyl group of catecholamines.

Conversion of murine Fabs isolated from a combinatorial phage display library to full length immunoglobulins.

The use of combinatorial Ig libraries displayed on the surface of bacteriophage has advantages over traditional hybridoma techniques for the generation of mAbs but in many instances full length Igs may be more desirable than Fab fragments. Two murine Fabs reactive with the human complement component C5a, recovered from a combinatorial library, were converted to full length IgG2a mAbs. The VH and VL domains of these antibodies were removed from the bacterial expression vector used for the combinatorial library construction, and subcloned into individual mammalian expression vectors containing the corresponding Ig heavy and light chain constant regions. The subcloning relied on 5' restriction endonuclease sites encoded by the oligonucleotide primers originally used to amplify the Ig cDNAs and 3' sites conserved in CH1 and C kappa. These vectors were co-transfected into COS cells yielding full length IgG2a versions of the anti-C5a antibodies. The mAbs, purified from the culture supernatant, retained the full activity of the Fabs, binding specifically to and neutralizing human recombinant C5a. Refined versions of the mammalian expression vectors have been constructed for single step conversion of murine recombinant Fabs, recovered from combinatorial libraries, to IgG2a mAbs.

Oligomerization of a membrane protein correlates with its retention in the Golgi complex.

The first membrane-spanning domain (m1) of the M glycoprotein of avian coronavirus (formerly called E1) is sufficient to retain this protein in the cis-Golgi. When the membrane-spanning domain of a protein which is efficiently delivered to the plasma membrane (VSV G protein) is replaced with m1, the resulting chimera (Gm1) is retained in the Golgi (Swift, A. M., and C. E. Machamer. 1991. J. Cell Biol. 115:19-30). When assayed in sucrose gradients, we observed that Gm1 formed a large oligomer, and that much of this oligomer was SDS resistant and stayed near the top of the stacking gel of an SDS-polyacrylamide gel. The unusual stability of the oligomer allowed it to be detected easily. Gm1 mutants with single amino acid substitutions in the m1 domain that were retained in the Golgi complex formed SDS-resistant oligomers, whereas mutants that were rapidly released to the plasma membrane did not. Oligomerization was not detected immediately after synthesis of Gm1, but occurred gradually with a lag of approximately 10 min, suggesting that it is not merely aggregation of misfolded proteins. Furthermore, oligomerization did not occur under several conditions that block ER to Golgi transport. The lumenal domain was not required for oligomerization since another chimera (alpha m1G), where the lumenal domain of Gm1 was replaced by the alpha subunit of human chorionic gonadotropin, also formed an SDS-resistant oligomer, and was able to form hetero-oligomers with Gm1 as revealed by coprecipitation experiments. SDS resistance was conferred by the cytoplasmic tail of VSV G, because proteolytic digestion of the tail in microsomes containing Gm1 oligomers resulted in loss of SDS resistance, although the protease-treated material continued to migrate as a large oligomer on sucrose gradients. Interestingly, treatment of cells with cytochalasin D blocked formation of SDS-resistant (but not SDS-sensitive) oligomers. Our data suggest that SDS-resistant oligomers form as newly synthesized molecules of Gm1 arrive at the Golgi complex and may interact (directly or indirectly) with an actin-based cytoskeletal matrix. The oligomerization of Gm1 and other resident proteins could serve as a mechanism for their retention in the Golgi complex.

Retention of a cis Golgi protein requires polar residues on one face of a predicted alpha-helix in the transmembrane domain.

The first membrane-spanning domain (m1) of the model cis Golgi protein M (formerly called E1) from the avian coronavirus infectious bronchitis virus is required for targeting to the Golgi complex. When inserted in place of the membrane-spanning domain of a plasma membrane protein (vesicular stomatitis virus G protein), the chimeric protein ("Gm1") is retained in the Golgi complex of transfected cells. To determine the precise features of the m1 domain responsible for Golgi targeting, we produced single amino acid substitutions in m1 and analyzed their effects on localization of Gm1. Expression at the plasma membrane was used as the criterion for loss of Golgi retention. Rates of oligosaccharide processing were used as a measure of rate and efficiency of transport through the Golgi complex. We identified four uncharged polar residues that are critical for Golgi retention of Gm1 (Asn465, Thr469, Thr476, and Gln480). These residues line one face of a predicted alpha-helix. Interestingly, when the m1 domain of the homologous M protein from mouse hepatitis virus is inserted into the G protein reporter, the chimeric protein is not efficiently retained in the Golgi complex, but transported to the cell surface. Although it possesses three of the four residues we identified as important in the avian m1 sequence, other residues in the membrane-spanning domain from the mouse protein must prevent efficient recognition of the polar face within the lipid bilayer of the cis Golgi.

A Golgi retention signal in a membrane-spanning domain of coronavirus E1 protein.

The E1 glycoprotein from an avian coronavirus is a model protein for studying retention in the Golgi complex. In animal cells expressing the protein from cDNA, the E1 protein is targeted to cis Golgi cisternae (Machamer, C. E., S. A. Mentone, J. K. Rose, and M. G. Farquhar. 1990. Proc. Natl. Acad. Sci. USA. 87:6944-6948). We show that the first of the three membrane-spanning domains of the E1 protein can retain two different plasma membrane proteins in the Golgi region of transfected cells. Both the vesicular stomatitis virus G protein and the alpha-subunit of human chorionic gonadotropin (anchored to the membrane by fusion with the G protein membrane-spanning domain and cytoplasmic tail) were retained in the Golgi region of transfected cells when their single membrane-spanning domains were replaced with the first membrane-spanning domain from E1. Single amino acid substitutions in this sequence released retention of the chimeric G protein, as well as a mutant E1 protein which lacks the second and third membrane-spanning domains. The important feature of the retention sequence appears to be the uncharged polar residues which line one face of a predicted alpha helix. This is the first retention signal to be defined for a resident Golgi protein. The fact that it is present in a membrane-spanning domain suggests a novel mechanism of retention in which the membrane composition of the Golgi complex plays an instrumental role in retaining its resident proteins.

Phosphorylation of a Mr 70,000 protein is associated with interleukin 2 receptor expression.

The human T cell hybrid II23 was isolated from fusions between human peripheral blood lymphocytes which had been stimulated with phytohemagglutinin (PHA) and a subline of the human T cell line CEM called CEM.TET1. This hybrid does not constitutively express detectable levels of interleukin 2 (IL 2) receptors but can be induced to express receptors by stimuli shown to activate T cells. Antibody to CD3 (a component of the T cell receptor) coupled to agarose or PHA (greater than 3 micrograms/ml) induced both IL 2 production and receptor expression on II23 cells. Phorbol 12-myristate 13-acetate (PMA) induced IL 2 receptor expression on II23 cells but not IL 2 secretion. Because PMA is a known activator of the Ca2+/phospholipid-dependent enzyme protein kinase C, proteins of stimulated and unstimulated cells were analyzed by two-dimensional gel electrophoresis for changes in phosphoprotein patterns. A Mr 70,000 protein with a pI of 6.2 was phosphorylated in hybrids stimulated by PMA, anti-CD3 antibody coupled to agarose or PHA, i.e. by the same stimuli which induce IL 2 receptors on these cells. The immunosuppressive drug cyclosporin A inhibited IL 2 release without altering induction of IL 2 receptors or phosphorylation of the Mr 70,000 protein. The 70-kDa protein was located in the cytosol, where it remained phosphorylated for at least 4 h after stimulation. A protein with the same migratory properties on two-dimensional gels was similarly phosphorylated after stimulation of normal peripheral blood T lymphocytes, indicating that the phenomenon was not due to hybridization or transformation. This 70-kDA protein may therefore be involved in the pathway which leads to the transcription and expression of IL 2 receptors.

Proacrosin conversion inhibitor. Purification and initial characterization of a boar sperm protein which prevents the conversion of proacrosin into acrosin.

A proacrosin conversion inhibitor present in boar spermatozoa has been purified and initially characterized. Purification methods included sequential acid extractions of washed spermatozoa at pH 4.0, pH 3.5, and pH 2.5 followed by successive gel filtrations of the pH 2.5 sperm extract supernatant over Sephadex G-75 and G-50. The resulting 8.8-fold purified materials were judged to be homogeneous by sodium dodecyl sulfate-polyacrylamide disc gel electrophoresis, had an estimated molecular weight of 12,800, and a constant specific activity of 65 units/mg. Treatment with the proteinases acrosin, trypsin, or chymotrypsin destroyed the highly purified proacrosin conversion inhibitor, indicating that it is a protein. Additional properties of the inhibitor included stability to long periods of storage at pH 3.0 and 4 degrees C, stability to boiling and lyophilization, and an absolute requirement for divalent cations to maintain activity. The highly purified proacrosin conversion inhibitor does not inhibit acrosin. Therefore, it apparently acts to prevent proacrosin conversion by selectively inhibiting the zymogen's self-catalyzed conversion mechanism.