CHARACTERIZATION AND QUANTITATION OF 3 B-TYPE LAMINS IN XENOPUS OOCYTES AND EGGS - INCREASE OF LAMIN L(I) PROTEIN-SYNTHESIS DURING MEIOTIC MATURATION

We have previously shown that Xenopus oocytes, eggs, and early embryos contain lamins L(II) and L(III), and that portions of each are associ ated with distinct egg vesicle populations. We now report that a lamin similar or identical to the B-type lamin L(I) is also present in oocy te nuclei and in egg extracts, We have quantitated the three B-type la mins per oocyte nucleus, and have calculated relative ratios of L(I):L (III) = 1:100, and L(II):L(III) = 1:10. Similar to lamin L(II), 5-15% of lamin L(I) is associated with egg membranes in a biochemically stab le manner, Egg vesicles absorbed with lamin isoform-specific antibodie s to magnetic beads indicate that lamin L(I)-associated egg membranes are of heterogenous morphology, and are independent from the lamin L(I I) and L(III) vesicle populations. Compared to other nuclear envelope proteins, the synthesis of lamin L(I) protein is specifically elevated during meiotic maturation, resulting in a 4- to 12-fold higher amount of lamin L(I) in eggs than is present in oocyte nuclei. Immunofluores cence and immunoblot analysis demonstrated that lamins L(I), L(II), an d L(III) are associated with the nuclear envelope formed on demembrana ted sperm when added to activated egg extract. These results strongly suggest that three different lamin-associated vesicle populations are involved in the formation of a nuclear envelope in egg extracts.