The Major 67 000 Molecular Weight Protein Of The Clam Oocyte Nuclear Envelope Is Lamin-Like

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Abstract
Nuclear envelopes of somatic cells have at least 2 different major proteins in the 60-70 (.times. 103) MW range (lamins A(C) and B) that seem to be involved in chromatin attachment. In contrast, nuclear envelopes from clam germinal vesicles have only a single major protein of the same size class (.apprx. 67 .times. 103 MW) and have no chromatin attached to them. This 67 .times. 103 MW clam protein shares a variety of physical properties with lamins A(C) and B, derived from rat liver nuclei. These properties include similar size, although different isoelectric points; phosphorylated forms; strong tendencies to cross-link by disulfide bonds; presence of carbohydrates, demonstrated by direct incorporation of mannose and labeling with borohydride; and shared epitopes, demonstrated using both monoclonal and polyclonal antibodies. Taken together, these observations identify the clam 67 .times. 103 MW protein, the major structural protein of a nuclear envelope that lacks attached chromatin, as being lamin-like and demonstrate that it is more closely related to lamin A(C) than to lamin B.