PKC-mediated phosphorylation of nuclear lamins at a single serine residue regulates interphase nuclear size in Xenopus and mammalian cells

How nuclear size is regulated is a fundamental cell-biological question with relevance to cancers, which often exhibit enlarged nuclei. We previously reported that conventional protein kinase C (cPKC) contributes to nuclear size reductions that occur during early Xenopus development. Here we report that PKC-mediated phosphorylation of lamin B3 (LB3) contributes to this mechanism of nuclear size regulation. By mapping PKC phosphorylation sites on LB3 and testing the effects of phosphomutants in Xenopus laevis embryos, we identify the novel site S267 as being an important determinant of nuclear size. Furthermore, FRAP studies demonstrate that phosphorylation at this site increases lamina dynamics, providing a mechanistic explanation for how PKC activity influences nuclear size. We subsequently map this X. laevis LB3 phosphorylation site to a conserved site in mammalian lamin A (LA), S268. Manipulating PKC activity in cultured mammalian cells alters nuclear size, as does expression of LA-S268 phosphomutants. Taken together, these data demonstrate that PKC-mediated lamin phosphorylation is a conserved mechanism of nuclear size regulation.