Abstract
The Drosophila hook gene encodes a novel component of the endocytic compartment. Previously identified hook alleles, which still expressed truncated Hook proteins, affected the accumulation of internalized transmembrane ligands into multivesicular bodies (MVBs). To determine the hook null phenotype, we isolated nine new hook alleles on the basis of their characteristic hooked-bristle phenotype. At least one of these alleles, hk11, is a complete loss-of-function allele. Flies carrying the hk11 allele are viable and fertile but neither transmembrane ligands nor soluble ligands accumulate in MVBs. This effect on endocytosed ligands can be mimicked by the expression of Hook proteins truncated for the N- and C-terminal domains flanking the central coiled-coil region. The importance of all three domains for Hook function was confirmed by their conservation between two Drosophila and two human Hook proteins.