When stimulated with antigen, B cells are influenced by T cells to proliferate and differentiate into antibody-forming cells. Since it was reported that soluble factors could replace certain functions of helper T cells in the antibody response, several different kinds of lymphokines and monokines have been reported in B-cell growth and differentiation. Among these, human B-cell differentiation factor (BCDF or BSF-2) has been shown to induce the final maturation of B cells into immunoglobulin-secreting cells. BSF-2 was purified to homogeneity and its partial NH2-terminal amino-acid sequence was determined. These studies indicated that BSF-2 is functionally and structurally unlike other known proteins. Here, we report the molecular cloning, structural analysis and functional expression of the cDNA encoding human BSF-2. The primary sequence of BSF-2 deduced from the cDNA reveals that BSF-2 is a novel interleukin consisting of 184 amino acids.

A gene from Saccharomyces cerevisiae has been mapped, cloned, sequenced and shown to encode a catalytic subunit of an N‐terminal acetyltransferase. Regions of this gene, NAT1, and the chloramphenicol acetyltransferase genes of bacteria have limited but significant homology. A nat1 null mutant is viable but exhibits a variety of phenotypes, including reduced acetyltransferase activity, derepression of a silent mating type locus (HML) and failure to enter G0. All these phenotypes are identical to those of a previously characterized mutant, ard1. NAT1 and ARD1 are distinct genes that encode proteins with no obvious similarity. Concomitant overexpression of both NAT1 and ARD1 in yeast causes a 20‐fold increase in acetyltransferase activity in vitro, whereas overexpression of either NAT1 or ARD1 alone does not raise activity over basal levels. A functional iso‐1‐cytochrome c protein, which is N‐terminally acetylated in a NAT1 strain, is not acetylated in an isogenic nat1 mutant. At least 20 other yeast proteins, including histone H2B, are not N‐terminally acetylated in either nat1 or ard1 mutants. These results suggest that NAT1 and ARD1 proteins function together to catalyze the N‐terminal acetylation of a subset of yeast proteins.

SummaryMethionine aminopeptidases with a universal specif city have been revealed from the sequences of the amino-terminal region of mutant forms of yeast iso-lcytochrome c and from a systematic examination of the literature for aminoterminal sequences formed at initiation sites. The aminopeptidase removes aminoterminal residues of methionine when they precede certain amino acids, with a specijicity that appears to be determined mainly by the residue adjacent to the methionine residue at the amino terminus. The result with the mutationally altered iso-1-cytochromes c and the results from published sequences of other proteins from a wide range of prokaryotes and eukaryotes suggest that the aminopeptidase usually cleaves amino-terminalmethionine when it precedes residues of alanine, cysteine, giycine, proline, serine, threonine and valine but not when it precedes residues of arginine, asparagine, aspartic acid, glutamine, glutamic acid, isoleucine, leucine, lysine or methionine. We suggest that the specy5city is almost always determined simply by the size of the side chain of the penultimate residue; methionine is mually cleavedfrom residues with a side chain having a radius of gyration of 1-29,d or less, but is not cleaved from residues with larger side chains.