PROBABLE MECHANISMS UNDERLYING INTERALLELIC COMPLEMENTATION AND TEMPERATURE-SENSITIVITY OF MUTATIONS AT THE SHIBIRE LOCUS OF DROSOPHILA-MELANOGASTER

The shibire locus of Drosophila melanogaster encodes dynamin, a GTPase required for the fission of endocytic vesicles from plasma membrane. Biochemical studies indicate that mammalian dynamin is part of a compl ex containing multiple dynamin subunits and other polypeptides. To gai n insight into sequences of dynamin critical for its function, we have characterized in detail a collection of conditional and lethal shi al leles. We describe a probable null allele of shi and show that its pro perties are distinct from those of two classes of lethal alleles (term ed I and II) that show intergroup, interallelic complementation. Seque nced class I alleles, which display dominant properties, carry missens e mutations in conserved residues in the GTPase domain of dynamin. In contrast, the sequenced class LT alleles, which appear completely rece ssive, carry missense mutations in conserved residues of a previously uncharacterized ''middle domain'' that lies adjacent to the GTPase reg ion. These data suggest that critical interactions mediated by this mi ddle domain are severely affected by the class II lethal mutations; th us, the mutant sequences should be very useful for confirming the in v ivo relevance of interactions observed in vitro. Viable heteroallelic combinations of shi lethals show rapid and reversible temperature-sens itive paralytic phenotypes hitherto only described for the ts alleles of shi. When taken together with the molecular analysis of shi mutatio ns, these observations suggest that the GTPase domain of dynamin carri es an intrinsically temperature-sensitive activity: hypomorphic mutati ons that reduce this activity at low temperatures result in conditiona l temperature-sensitive phenotype. These observations explain why scre ens for conditional paralytic mutants in Drosophila inevitably recover ts alleles of shi at high frequencies.