Nuclear Envelope Proteins: Chemical Characteristics and Molecular Interactions of the Lamins

The lamins are a group of proteins in a residual nuclear envelope fraction derived from the nuclear lamina. The absence of specific functional assays for the two predominant species, lamin A and lamin B, has complicated their biochemical characterization. Initial studies reported in this work involved the enzymic cleavage of isolated lamins A and B and another lamina protein with a molecular weight of 61,000-daltons. The results obtained with each of three enzymes indicated strong similarities between the three nuclear envelope proteins. Further chemical characterization of the lamins has been achieved. Lamins A, B and C have been separated by two-dimensional electrophoresis based on their isoelectric point and molecular weight. Two-dimensional 125 I-tryptic peptide maps were prepared. The results suggest that lamin C is a cleavage product of lamin A, while lamin B has a distinct primary sequence. The topography of the lamins in the isolated nuclear envelope has been examined. Both lamin A and lamin B can be converted to homogeneous polymers by oxidation of intrinsic sulfhydryl groups with o-phenanthroline cupric ion complex. The pattern of polymers formed by oxidative cross-linking was different for lamin A and lamin B. The nature of these homogeneous polymers was investigated further. The crosslinked polymers of lamin A included dimers, trimers, tetramers and larger oligomers. Those that entered the acrylamide gels migrated as expected for their molecular weights. The migration of lamin B oligomers has been found to vary with electrophoretic conditions and this has led to a controversy as to the number of chains in the lamin B oligomers. Three lamin B crosslinked oligomers have been detected and most likely represent a dimer, a trimer and a tetramer. Evidence for this model has been obtained by analysis of cross-linked fragments from a mild trypsin digestion of oxidized lamina proteins. This procedure yields 62,000-, 46,000- and 30,000-dalton fragments of lamin B. Each fragment also occurs as three homotypic oligomers. A further study involved a high resolution two-dimensional gel electrophoretic system for separating the lamina proteins. Using this system a subtype of lamin B has been found in the avian erythrocyte. This subtype, called lamin Bl, is slightly larger and more acidic than the quantitatively major subtype now called lamin B2. The lamin B subtypes have very similar primary sequences and share a distinctive topography