ABSTRACT
The lymphokine tumor necrosis factor (TNF) has a well-defined role as an inducer of inflammatory responses; however, the function of the structurally related molecule lymphotoxin (LT alpha) is unknown. LT alpha is present on the surface of activated T, B, and LAK cells as a complex with a 33 kd glycoprotein, and cloning of the cDNA encoding the associated protein, called lymphotoxin beta (LT beta), revealed it to be a type II membrane protein with significant homology to TNF, LT alpha, and the ligand for the CD40 receptor. The gene for LT beta was found next to the TNF-LT locus in the major histocompatibility complex (MHC), a region of the MHC with possible linkage to autoimmune disease. These observations raise the possibility that a surface LT alpha-LT beta complex may have a specific role in immune regulation distinct from the functions ascribed to TNF.
Subject(s)
Lymphotoxin-alpha/chemistry , Lymphotoxin-alpha/metabolism , Amino Acid Sequence , Base Sequence , Chromosomes, Human, Pair 6 , Cloning, Molecular , Gene Expression , Genes , Humans , Lymphotoxin-alpha/genetics , Macromolecular Substances , Major Histocompatibility Complex , Molecular Sequence Data , Protein Conformation , RNA, Messenger/genetics , Sequence Alignment , Sequence Homology, Amino Acid , Transfection , Tumor Necrosis Factor-alpha/geneticsABSTRACT
We have cloned and sequenced mouse brain AP47, the medium chain of the trans-Golgi network clathrin-associated protein complex AP-1. The predicted protein sequence of AP47 is closely related to rat and calf brain AP50, the corresponding medium chain of the plasma-membrane clathrin-associated protein complex AP-2. We have also identified in the yeast genome an open reading frame encoding a protein of previously unknown function. Referred to here as YAP54, its predicted protein sequence displays a striking homology to AP47. We therefore propose that Yap54 is the medium chain subunit of a putative AP-1 complex in yeast. From the analyses of the optimized sequence alignments of AP47, AP50 and Yap54p, we suggest a model for the domain organization of the medium chains.
Subject(s)
Adaptor Protein Complex 2 , Adaptor Protein Complex mu Subunits , Clathrin/metabolism , Fungal Proteins/genetics , Phosphoproteins/genetics , Saccharomyces cerevisiae/genetics , Adaptor Proteins, Vesicular Transport , Amino Acid Sequence , Animals , Base Sequence , Blotting, Southern , Cattle , Cloning, Molecular , DNA/genetics , Fungal Proteins/analysis , Fungal Proteins/chemistry , Molecular Sequence Data , Phosphoproteins/analysis , Phosphoproteins/chemistry , Rats , Saccharomyces cerevisiae/chemistry , Sequence AlignmentABSTRACT
We have characterized a 68 kDa lipocortin from human placenta that was identified as a covalently linked homodimer of lipocortin-1 by peptide mapping and sequence analysis. The site of cross-linking was localized within the 3 kDa N-terminal tail region, an exposed domain that contains the phosphorylation sites for protein tyrosine kinase and protein kinase C and is sensitive to proteolysis. Sequence analysis of the corresponding peptide revealed that glutamine-18 was modified, suggesting that the cross-link may be generated by a transglutaminase. By incubating lipocortin-1 with placental membranes and with labelled glycine ethyl ester we observed a Ca2+-dependent labelling of lipocortin-1 within the tail region, supporting this notion. Like lipocortin-1, the dimer inhibits phospholipase Ad2 activity, is a substrate for the epidermal-growth-factor (EGF) receptor/kinase, and display Ca2+-dependent binding to phosphatidylserine-containing vesicles. In preparations from human placenta the dimer is particularly abundant, accounting for approx. 20% of the lipocortin-1.
