MEMBRANE-ASSOCIATED LAMINS IN XENOPUS EGG EXTRACTS - IDENTIFICATION OF 2 VESICLE POPULATIONS

Nuclear lamin isoforms of vertebrates can be divided into two major cl asses. The B-type lamins are membrane associated throughout the cell c ycle, whereas A-type lamins are recovered from mitotic cell homogenate s in membrane-free fractions. A feature of oogenesis in birds and mamm als is the nearly exclusive presence of B-type lamins in oocyte nuclea r envelopes. In contrast, oocytes and early cleavage embryos of the am phibian Xenopus laevis are believed to contain a single lamin isoform, lamin L(III), which after nuclear envelope breakdown during meiotic m aturation is reported to be completely soluble. Consequently, we have reexamined the lamin complement of Xenopus oocyte nuclear envelopes, e gg extracts, and early embryos. An mAb (X223) specific for the homolog ous B-type lamins B2 of mouse and L(II) of Xenopus somatic cells (Hoge r, T., K. Zatloukal, I. Waizenegger, and G. Krohne. 1990. Chromosoma. 99:379-390) recognized a Xenopus oocyte nuclear envelope protein bioch emically distinct from lamin L(III) and very similar or identical to s omatic cell lamin L(II). Oocyte lamin L(II) was detectable in nuclear envelopes of early cleavage embryos. Immunoblotting of fractionated eg g extracts revealed that approximately 20-23% of lamin L(II) and 5-7% of lamin L(III) were membrane associated. EM immunolocalization demons trated that membrane-bound lamins L(II) and L(III) are associated with separate vesicle populations. These findings are relevant to the inte rpretation of nuclear reconstitution experiments using Xenopus egg ext racts.