A second higher vertebrate B-type lamin. cDNA sequence determination and in vitro processing of chicken lamin B2

The chicken nuclear lamina is composed of at least three proteins called lamins A, B1 and B2. In addition, putative precursors are transiently expressed during in vivo synthesis of lamins A and B2. Here we report the complete sequence of lamin B2 as it is deduced from a cloned cDNA. Comparison of lamin B2 with lamins A and B1 in the accompanying paper provides definitive proof for the existence of two structurally distinct chicken B-type lamins. Furthermore, we show that in vitro translation of transcripts derived from lamin A and lamin B2 cDNAs yielded polypeptides that were indistinguishable, by two-dimensional gel electrophoresis, from the putative in vivo precursors of lamins A and B2 respectively. However, whereas the lamin A precursor was stable, the translation product of the lamin B2 transcript was processed in the reticulocyte lysate to a polypeptide comigrating on two-dimensional gels with authentic mature lamin B2. This processing event could be inhibited by chelators of divalent cations, i.e. o-phenanthroline and EDTA. Our results indicate that the transiently expressed variant of lamin B2 represent a bonafide precursor, and that two distinct activities are involved in processing of newly synthesized lamins A and B2. Lamin precursors processing is discussed in relation to characteristic differences in the interactions of A and B-type lamins with the nuclear membrane.