Substrate selection and processing in endocytosis.

In an earlier contribution to these Transactions (Lloyd & Williams, 1984) the characteristics of non-specific adsorptive pinocytosis were discussed. Evidence was presented that many macromolecules captured by adsorptive pinocytosis display either cationic or hydrophobic determinants, and that many types of cell possess surface features that interact positively with such ligands. Internalization of these macromolecules is a consequence of the constant formation and inflow of pinocytic vesicles. Thus constitutive pinocytosis has a high degree of inbuilt substrate specificity and there is no need, in most instances of adsorptive uptake, to invoke a specific substrate-triggered pinocytic event. The earlier article also discussed critically the techniques available for quantifying substrate uptake by pinocytosis. In this second contribution we review our recent work on the substrate specificity of pinocytosis and the subsequent processing of internalized macromolecules within the lysosomes. A common theme of most of this work is the use of synthetic polymers as custom-designed probes for these cellular phenomena.