B. Messmer et C. Dreyer, REQUIREMENTS FOR NUCLEAR TRANSLOCATION AND NUCLEOLAR ACCUMULATION OF NUCLEOLIN OF XENOPUS-LAEVIS, European journal of cell biology, 61(2), 1993, pp. 369-382
The intracellular localization of germinal vesicle proteins is regulat
ed during early Xenopus development. Here we have analyzed the determi
nants that control the localization of nucleolin, a nucleolar protein
of vertebrates. Our immunological analyses and isolation of cDNAs have
revealed the presence of a second nucleolin gene in addition to that
previously identified. The two nucleolin polypeptides of 95 kDa and 90
kDa molecular mass are both expressed in oocytes and are found predom
inantly in the nucleoli. During oocyte maturation, both polypeptides a
re hyper-phosphorylated and found distributed throughout the cytoplasm
. Hyperphosphorylated forms and cytoplasmic localization of nucleolin
both prevail in early embryos up to the midblastula transition. Subseq
uently, the proteins are found in the nuclei. Accumulation in nucleoli
is only detected at later stages, beginning with gastrulation. Thus n
ucleolar localization is temporally uncoupled from nuclear translocati
on. Consistent with these observations, our molecular analyses have re
vealed that the nuclear location signal which is present in nucleolin
and which is sufficient for nuclear location, is not sufficient for nu
cleolar accumulation. Nucleolar accumulation requires, in addition to
the nuclear location signal, the presence of the RNA binding domains a
nd of an RG-rich domain, which is also thought to interact with RNA.