Determinants for activation of the atypical AGC kinase Greatwall during M phase entry.

The atypical AGC kinase Greatwall (Gwl) mediates a pathway that prevents the precocious removal of phosphorylations added to target proteins by M phase-promoting factor (MPF); Gwl is thus essential for M phase entry and maintenance. Gwl itself is activated by M phase-specific phosphorylations that are investigated here. Many phosphorylations are nonessential, being located within a long nonconserved region, any part of which can be deleted without effect. Using mass spectrometry and mutagenesis, we have identified 3 phosphorylation sites (phosphosites) critical to Gwl activation (pT193, pT206, and pS883 in Xenopus laevis) located in evolutionarily conserved domains that differentiate Gwl from related kinases. We propose a model in which the initiating event for Gwl activation is phosphorylation by MPF of the proline-directed sites T193 and T206 in the presumptive activation loop. After this priming step, Gwl can intramolecularly phosphorylate its C-terminal tail at pS883; this site probably plays a role similar to that of the tail/Z motif of other AGC kinases. These events largely (but not completely) explain the full activation of Gwl at M phase.

The M phase kinase Greatwall (Gwl) promotes inactivation of PP2A/B55delta, a phosphatase directed against CDK phosphosites.

We have previously shown that Greatwall kinase (Gwl) is required for M phase entry and maintenance in Xenopus egg extracts. Here, we demonstrate that Gwl plays a crucial role in a novel biochemical pathway that inactivates, specifically during M phase, "antimitotic" phosphatases directed against phosphorylations catalyzed by cyclin-dependent kinases (CDKs). A major component of this phosphatase activity is heterotrimeric PP2A containing the B55delta regulatory subunit. Gwl is activated during M phase by Cdk1/cyclin B (MPF), but once activated, Gwl promotes PP2A/B55delta inhibition with no further requirement for MPF. In the absence of Gwl, PP2A/B55delta remains active even when MPF levels are high. The removal of PP2A/B55delta corrects the inability of Gwl-depleted extracts to enter M phase. These findings support the hypothesis that M phase requires not only high levels of MPF function, but also the suppression, through a Gwl-dependent mechanism, of phosphatase(s) that would otherwise remove MPF-driven phosphorylations.

Interaction of a C-terminal peptide of Bos taurus diacylglycerol acyltransferase 1 with model membranes.

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final and dedicated step in the synthesis of triacylglycerol, which is believed to involve the lipids oleoyl coenzyme A (OCoA) and dioleoyl-sn-glycerol (DOG) as substrates. In this work we investigated the interaction of a specific peptide, referred to as SIT2, on the C-terminal of DGAT1 (HKWCIRHFYKP) with model membranes made with OCoA and DOG in Langmuir monolayers and liposomes. According to the circular dichroism and fluorescence data, conformational changes on SIT2 were seen only on liposomes containing OCoA and DOG. In Langmuir monolayers, SIT2 causes the isotherms of neat OCoA and DOG monolayers to be expanded, but has negligible effect on mixed monolayers of OCoA and DOG. This synergistic interaction between SIT2 and DOG+OCoA may be rationalized in terms of a molecular model in which SIT2 may serve as a linkage between the two lipids. Our results therefore provide molecular-level evidence for the interaction between this domain and the substrates OCoA and DOG for the synthesis of triacylglycerol.

Isolation and characterization of four type 2 ribosome inactivating pulchellin isoforms from Abrus pulchellus seeds.

Abrus pulchellus seeds contain at least seven closely related and highly toxic type 2 ribosome-inactivating pulchellins, each consisting of a toxic A-chain linked to a sugar binding B-chain. In the present study, four pulchellin isoforms (termed P I, P II, P III and P IV) were isolated by affinity, ion exchange and chromatofocusing chromatographies, and investigated with respect to toxicity and sugar binding specificity. Half maximal inhibitory concentration and median lethal dose values indicate that P I and P II have similar toxicities and that both are more toxic to cultured HeLa cells and mice than P III and P IV. Interestingly, the secondary structural characteristics and sugar binding properties of the respective pairs of isoforms correlate well with the two toxicity levels, in that P I/P II and P III/P IV form two specific subgroups. From the deduced amino acids sequences of the four isoforms, it is clear that the highest similarity within each subgroup is found to occur within domain 2 of the B-chains, suggesting that the disparity in toxicity levels might be attributed to subtle differences in B-chain-mediated cell surface interactions that precede and determine toxin uptake pathways.

Endocytosis of pulchellin and its recombinant B-chain into K-562 cells: binding and uptake studies.

Most of the type 2 ribosome-inactivating proteins (RIPs) are toxins formed by an RNA-N-glycosidase A-chain polypeptide linked to a lectin B-chain by a single disulfide bond. Members of this protein class vary greatly in cytotoxity, correlating more with B-chain diversity rather than to A-chain differences. Pulchellin is a type 2 ribosome-inactivating protein toxin found in the seeds of Abrus pulchellus tenuiflorus. Recombinant pulchellin B-Chain (rPBC) has been previously produced as inclusion bodies in Escherichia coli and successfully refolded recovering biological activity. New approaches for using this kind of protein as a biotechnological tool require a better understanding of cell targeting, binding, uptake, intracellular routing and delivery. In this work, cell adhesion experiments were used to determine the interaction of rPBC with mammalian cells. Fluorescence and confocal microscopy revealed the intracellular localization and trafficking. Subcellular sorting of the native pulchellin could also be determined. The results support that the endosomal internalization pathway and the retrograde transport through the Golgi apparatus might be used by both native protein and rPBC.

Heterologous expression, characterization and structural studies of a hydrophobic peptide from the HIV-1 p24 protein.

Proteins from the inner core of HIV-1, such as the capsid protein (p24), are involved in crucial processes during the virus life cycle. The p24 protein plays an active structural role in the Gag protein and in its mature form. This work describes the production of a peptide derived from the p24 C-terminal, TLRAEQASQEVKNWMTETLLVQNA, using recombinant technology. This region (p24-3) is involved in interfaces during the p24 dimerization, which occurs during capsid assembly. The p24-3 sequence was obtained by a synthetic gene strategy and inserted into the pET 32a expression vector to produce soluble fusion protein in Escherichia coli BL21(DE3). This strategy leads to an incorporation of three amino acid residues (AMA) in the N-terminal of the native sequence to form the recombinant p24-3 (rp24-3). The rp24-3 was purified by reverse phase chromatography to homogeneity, as inferred by mass spectrometry and protein sequence analysis. Structural studies using circular dichroism and steady-state fluorescence showed that the rp24-3 is structured by helical and beta elements. As a function of its hydrophobic character it can self-associate forming oligomers. We present in this paper the first development of a suitable expression system for rp24-3, which provides high amounts of the peptide. This strategy will allow the development of new antiviral (HIV) agents.