ENTRY OF EXOGENOUS POLYPEPTIDES INTO THE NUCLEUS OF LIVING CELLS - FACTS AND SPECULATIONS

Although the plasma membrane acts as art impermeable barrier to most m acromolecules, some exogenous proteins (for example fibroblast growth factor, HIV-1 Tat and lactoferrin) can gain access into the cytosol an d reach the nucleus of living cells. How are these exogenous polypepti des selected over and above other extracellular proteins? How and wher e no they cross the cell membrane? Why no cells need to take up exogen ous transcription factors when sophisticated signal-transduction pathw ays are available? Here, we review the current knowledge on these issu es and discuss some mechanistic and physiological implications of this unconventional and direct way of taking messages to the nucleus.