The major 67 000 molecular weight protein of the clam oocyte nuclear envelope is lamin-like.

Nuclear envelopes of somatic cells have at least two different major proteins in the 60-70(X 10(3] molecular weight 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 (approximately 67 X 10(3) Mr) and have no chromatin attached to them. The data presented in this report show that this 67 X 10(3) Mr 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 disulphide bonds; presence of carbohydrates, demonstrated by direct incorporation of mannose and labelling with borohydride; and shared epitopes, demonstrated using both monoclonal and polyclonal antibodies. Taken together, these observations identify the clam 67 X 10(3) Mr 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.