The tetraspanins are integral membrane proteins expressed on cell surface
and granular membranes of hematopoietic cells and are components of multimolecular
complexes with specific integrins. The tetraspanin CD63 (also
known as LAMP-3, Melanoma-associated antigen ME491, TSPAN30, MLA1
and OMA81H) is a lysosomal membrane glycoprotein that translocates to the
plasma membrane after platelet activation. CD63 is expressed on activated
platelets, monocytes and macrophages, and is weakly expressed on granulocytes,
T cell and B cells. It is located on the basophilic granule membranes and
on the plasma membranes of lymphocytes and granulocytes. CD63 is a member
of the TM4 superfamily of leukocyte glycoproteins that includes CD9,
CD37 and CD53, which contain four transmembrane regions. CD63 may play
a role in phagocytic and intracellular lysosome-phagosome fusion events.
CD63 deficiency is associated with Hermansky-Pudlak syndrome.