Although arginine modification has been implicated in a number of cellular processes, the in vivo requirement of protein arginine methyltransferases (PRMTs) in specific biological processes remain to be clarified. In this study we characterize the Drosophila PRMT Capsuléen, homologous to human PRMT5. During Drosophila oogenesis, catalytic activity of Capsuléen is necessary for both the assembly of the nuage surrounding nurse cell nuclei and the formation of the pole plasm at the posterior end of the oocyte. In particular, we show that the nuage and pole plasm localization of Tudor, an essential component for germ cell formation, are abolished in csul mutant germ cells. We identify the spliceosomal Sm proteins as in vivo substrates of Capsuléen and demonstrate that Capsuléen, together with its associated protein Valois, is essential for the synthesis of symmetric di-methylated arginyl residues in Sm proteins. Finally, we show that Tudor can be targeted to the nuage in the absence of Sm methylation by Capsuléen, indicating that Tudor localization and Sm methylation are separate processes. Our results thus reveal the role of a PRMT in protein localization in germ cells.

The genes torso (tor) and torso-like (tsl) are two of the Drosophila maternal group genes implicated in a receptor tyrosine kinase signalling pathway that specifies terminal cell fate (reviewed in ref. 3). Loss-of-function mutations in these loci cause an identical phenotype in which pattern elements from the anterior (acron) and posterior (telson) ends have been deleted. We have cloned the tsl gene and demonstrate here that, in agreement with previous genetic data, it encodes a protein that is secreted and whose transcription is restricted to specialized categories of follicle cells localized at the poles of the egg chamber. At early blastoderm stage, tsl protein forms a symmetrical concentration gradient at the poles on the surface of the devitellinized embryo. Unrestricted expression of the tsl protein in tsl female mutants induces terminal pattern elements and suppresses the formation of abdomen in embryos. These results suggest that the tsl protein is the ligand that binds to the torso receptor.

Calcium/calmodulin‐dependent protein kinases (CaM kinases) have been reported to be involved in neuroplasticity. We have cloned a new Drosophila CaM kinase gene named caki. We describe the molecular characterization of caki and a behavioral effect of its elimination. The caki gene is extremely large; comparison of the genomic and cDNA sequences reveals that the caki transcription unit is at least 150 kb. The catalytic domain of this new CaM kinase protein shares homology (41%) with type II CaM kinases, while the C‐terminal part is divergent. Constitutively expressed Caki protein is enzymatically active since it causes a 3‐fold increase in the level of the Rous sarcoma virus long terminal repeat (RSV LTR) promoter in a co‐transfusion assay. In situ hybridization shows that during embryogenesis, larval and pupal life, transcription of caki is restricted almost exclusively to the central nervous system. In the adult head, immunohistochemistry reveals Caki protein in the lamina, the neuropil of the medulla, lobula, lobula plate and in the central brain. Mutant caki flies show reduced walking speed in ‘Buridan's paradigm’.