Internalization of macromolecules by live cells

The membrane of eukaryotic cells represents a barrier normally impermeable to non-lipophilic substances. In consequence, internalization of extracellular compounds requires the presence of specific channels, or of specialized endocytic mechanisms. Recently, several groups have reported that some growth factors and transcription factors can be internalized by cells in culture and addressed to the cytoplasmic and nuclear compartments. In the latter case, they have been shown to interfere directly with gene transcription. Among these factors are fibroblast growth factor-1 and -2 (FGF-1 and FGF-2), schwannoma-derived growth factor (SDGF), Tat protein and homeodomain or homeodomain containing polypeptides. In the present paper, we present the available data concerning these unconventional uptake mechanisms and speculate on their potential interest to target macromolec-ular structures into live cells.