Phosphorylation of p27^BBP/eIF6 and its association with the cytoskeleton are developmentally regulated in Xenopus oogenesis

p27^BBP/eIF6 is an evolutionarily conserved regulator of ribosomal function. It is necessary for 60S biogenesis and impedes improper joining of 40S and 60S subunits, regulated by protein kinase C or Efl1p. No data on p27^BBP/eIF6 during early development of Metazoa are available. We studied the distribution, post-translational changes and association with the cytoskeleton of p27^BBP/ eIF6 during Xenopus oogenesis and early development. Results indicate that p27^BBP/eIF6 is present throughout oogenesis, partly associated with 60S subunits, partly free and with little cytoskeleton bound. During prophase I, p27^BBP/eIF6 is detected as a single band of 27-kDa. Upon maturation induced by progesterone or protein kinase C, a serine-phosphorylated 29 kDa isoform appears and is kept throughout development to the neurula stage. Confocal microscopy showed that the distribution of p27^BBP/eIF6 and its association with the cytoskeleton varies according to oogenesis stages. Briefly, in stage 6 oocytes, p27 ^BBP/eIF6 has a limited dot-like distribution, and does not co-localize with cytokeratin, whereas upon maturation it spreads throughout the cytoplasm. After fertilization, a large fraction coalesces around cytomembranes and a cytochalasin B-sensitive co-localization with cytokeratin occurs. RNAse removes p27^BBP/eIF6 from the cytokeratin fibres. Developmental data suggest a role of p27^BBP/eIF6 in controlling ribosomal availability or regulating cross-talk between ribosomes and the cytoskeleton.