The role of the CD44 cytoplasmic and transmembrane domains in constitutiveand inducible hyaluronan binding

The role of the CD44 cytoplasmic domain in cells displaying constitutive or inducible hyaluronan (HA) binding mediated by wild-type CD44 was investiga ted using mutant CD44 constructs with the cytoplasmic domain truncated or r eplaced by foreign sequences. In cell lines in which wild-type CD44 bound H A constitutively, chimeric constructs consisting of the CD44 external domai n and the transmembrane plus cytoplasmic domains of beta 5 integrin bound a s well as wild-type CD44, arguing that the specific sequence of the cytopla smic and transmembrane domains was not critical for HA binding by the exter nal domain. The cytoplasmic domain sequence did not contribute to the 'indu cible' phenotype in cell lines which did not bind HA constitutively, but wh ich could be induced to bind by CD44-specific mAb. Tailless CD44 was induci ble in these cells, as was chimeric CD44 with the integrin beta 5 cytoplasm ic domain. Dimer- or oligomerization of CD44 by addition of AP1510 to cells containing CD44/FKBP chimeric constructs caused a modest enhancement of HA binding in cells that bound constitutively, but did not alter the inducibl e phenotype. This result suggests that clustering of CD44 from inside the c ell is not a sufficient 'inside-out' signal to activate HA binding in induc ible cells.