# School of Biological Sciences, University of Southampton, % Division of Molecular Genetics, University of Glasgow,
We have isolated and characterised the smallminded gene from Drosophila, encoding a new member of the AAA family of ATPases which includes NEM-sensitive fusion protein (NSF), transitional endoplasmic reticulum ATPase (TERA) and CDC48p. The predicted amino acid sequence contains a duplicated domain structure, characteristic of many of these proteins. Each domain is about 200 amino acids in length and contains nucleotide binding consensus sequences and a characteristic family consensus (AAA-motif). The greatest sequence homology is found with the final AAA member from S. cerevisiae (ORF YLLO34c from chromosome XII), the archaebacterial proteins SAV and CdcH, porcine VCP and its vertebrate higher plant and yeast homologues. smallminded shows 26-33% identity with these proteins. In common with the other members of this family the conserved regions of smallminded are found within and around the nucleotide binding domains. Here the aforementioned proteins show 41-48% and 53-60% identity for the first and second domains respectively. During embryogenesis smallminded RNA is expressed ubiquitously at high levels. However in the adult fly it is found to be expressed at high levels only in neurons and muscle cells. Flies homozygous for a null mutation of the smallminded gene exhibit weak larval locomotion and die as second instar larvae. Anatomically, mutant larvae are characterised by the size of the CNS which is significantly smaller than seen in wild type L2 larvae. This primary cause of this is failure of the larval neuroblasts to resume normal patterns of mitosis. Similar defects are seen in the imaginal disks which also fail to grow, consistent with arrested cell division. This evidence, together with the sequence homology between the predicted gene product and proteins known to be involved in membrane fusion events, suggests that the mutant phenotype may be caused by a deficiency in some aspect of its membrane fusion machinery relating to cell division.