The multiple myelin basic protein (MBP) isoforms expressed by myelinat
ing cells are now known to have different expression patterns, The rel
ative abundance of the isoforms containing exon II is greater early in
myelinogenesis, whereas in compact myelin the isoforms lacking this e
xon are more abundant, Further, the individual MBPs exhibit different
intracellular localizations, indicating that the isoforms may not be f
unctionally equivalent in myelinating cells, The major MBPs (14 kD and
18.5 kD) have strong affinity for membranes, while on the other hand,
the less abundant isoforms (17 kD and 21.5 kD) localize to the nucleu
s of young oligodendrocytes, suggesting a regulatory role in the myeli
nation program, The same intracellular distribution patterns have been
observed when the MBPs are expressed in Hela cells and in shiverer ol
igodendrocytes, Thus, the intracellular fate of these proteins seems t
o be generally directed through alternative expression of exon II, Fur
thermore, the extent of MBPexII entry into the nucleus was found to be
directly related to the growth state of host cells. In this paper, we
demonstrate that nuclear proteins constitutively expressed by Hela ce
lls also exhibit an apparently growth-related nucleo-cytoplasmic distr
ibution revealing that MBPexII exhibits the same behavior as bona fide
nuclear proteins, Also, to further characterize MBP nuclear transport
, we explored various parameters of the translocation of MBP into the
nucleus using an in vitro system, This experimental paradigm permits t
he uncoupling of synthesis and translocation events; thus, the transpo
rt of MBP into cell nuclei can be studied as a function of time, We al
so evaluated how changes in temperature as well as energy depletion af
fect the in vitro nuclear transport of MBP. (C) 1997 Wiley-Liss, Inc.