37] Ligand-induced patching and capping of surface immunoglobulins

Abstract

This chapter discusses the patching and capping phenomena that has been described for a variety of membrane components in a number of different cell types. The basic model remains, however, the patching and capping of membrane immunoglobulin (mIg) la of B lymphocytes. With other membrane components, the conditions for obtaining such redistribution phenomena may be significantly different. The chapter states that a detailed model for the mechanisms of patching and capping, and related membrane phenomena such as pinosome formation, microvilli formation, endocytosis, and shedding, has also been developed. The chapter reviews the general characteristics of the capping phenomenon, particularly the energy requirements, temperature dependence, inhibition, reversion by microfilament-directed drugs, and the accumulation of microfilaments under the cap that have led to the suggestion that capping is probably a cellular contractile phenomenon. The role of microtubules in the capping phenomenon is more complex and they do not appear to enhance microfilament mobility but rather to inhibit it. Capping of any membrane component probably occurs as a consequence of microprecipitation resulting in entrapment within the lattice formed in the plasma membrane, of a microfilament-associated membrane component: the “membrane capping component” or “integral protein X.” The hypothesis has been formulated, however, that not only clustered components but also whole domains of membrane might be brought into the cap.