CD44 structure and function.

In this review we discuss the structural elements of CD44 that have been shown to be involved in specific functions. To this end, we focus primarily on experiments in which CD44 constructs are transfected into cells whose function is then assayed. The hyaluronan binding function of CD44 has been assayed in cell lines and in fusion proteins, termed CD44-Igs, consisting of the external domain of CD44 coupled to the hinge, CH2 and CH3 regions of human IgG1. These studies have shown that hyaluronan binding by CD44 is regulated by the cells in which it is expressed, and that at least part of this regulation is determined by cell specific posttranslational modifications, especially N-glycosylation, of CD44 itself. Variant isoforms of CD44 determined by alternative splicing of 11 optional exons in the middle of the gene determine additional functions of CD44, as well as contributing to the regulation of hyaluronan binding. Soluble CD44 may modulate the function of cell surface CD44. The cytoplasmic domain of CD44 contributes to ligand binding in a way that remains obscure. It also determines membrane localization in polarized epithelial cells, and is probably involved in CD44 interactions with the cytoskeleton and in mediating post-ligand binding events.