Biogenesis of the nuclear lamina : in vivo synthesis and processing of nuclear protein precursors

Utilizing antibodies against lamins A, B1, and B2, we have studied the biogenesis of the nuclear lamina in chicken embryo fibroblasts. (Lamins B1 and B2 have been identified recently as structurally distinct "lamin B" proteins.) We demonstrate that, unique among the nuclear proteins studied to date, lamin A is synthesized as a higher molecular mass precursor. A short-lived higher molecular mass variant (t 1/2 approximately equal to 3 min) accompanying the mature-size protein was also detected in the case of lamin B2 biosynthesis, but no precursor was found for lamin B1. By combining pulse-chase experiments with subcellular fractionation, we provide evidence that synthesis of lamin proteins occurs on free polysomes; subsequently, the newly synthesized proteins become rapidly associated with a crude nuclear fraction. The lamin A precursor is processed within the nucleus with a half-time of about 30 min. Concomitantly, lamin proteins acquire a characteristic resistance to detergent extraction, suggesting their insertion into a submembraneous protein network. The described biogenetic pathway involving precursor synthesis and processing is very unusual for nuclear proteins; it may have interesting implications for the mechanisms of transport and assembly of poorly soluble nuclear proteins.