Vertebrate synapsins are abundant synaptic vesicle phosphoproteins that have been proposed to fine-regulate neurotransmitter release by phosphorylation-dependent control of synaptic vesicle motility. However, the consequences of a total lack of all synapsin isoforms due to a knock-out of all three mouse synapsin genes have not yet been investigated. In Drosophila a single synapsin gene encodes several isoforms and is expressed in most synaptic terminals. Thus the targeted deletion of the synapsin gene of Drosophila eliminates the possibility of functional knock-out complementation by other isoforms. Unexpectedly, synapsin null mutant flies show no obvious defects in brain morphology, and no striking qualitative changes in behaviour are observed. Ultrastructural analysis of an identified 'model' synapse of the larval nerve muscle preparation revealed no difference between wild-type and mutant, and spontaneous or evoked excitatory junction potentials at this synapse were normal up to a stimulus frequency of 5 Hz. However, when several behavioural responses were analysed quantitatively, specific differences between mutant and wild-type flies are noted. Adult locomotor activity, optomotor responses at high pattern velocities, wing beat frequency, and visual pattern preference are modified. Synapsin mutant flies show faster habituation of an olfactory jump response, enhanced ethanol tolerance, and significant defects in learning and memory as measured using three different paradigms. Larval behavioural defects are described in a separate paper. We conclude that Drosophila synapsins play a significant role in nervous system function, which is subtle at the cellular level but manifests itself in complex behaviour.

The vasodilator‐stimulated phosphoprotein (VASP), a substrate for cAMP‐ and cGMP‐dependent protein kinases in vitro and in intact cells, is associated with actin filaments, focal adhesions and dynamic membrane regions. VASP, cloned here from human HL‐60 and canine MDCK cells, is organized into three distinct domains. A central proline‐rich domain contains a GPPPPP motif as a single copy and as a 3‐fold tandem repeat, as well as three conserved phosphorylation sites for cyclic nucleotide‐dependent protein kinases. A C‐terminal domain contains a repetitive mixed‐charge cluster which is predicted to form an alpha‐helix. The hydrodynamic properties of purified human VASP together with the calculated molecular mass of cloned VASP suggest that the native protein is a homotetramer with an elongated structure. VASP over‐expressed in transiently transfected BHK21 cells was predominantly detected at stress fibres, at focal adhesions and in F‐actin‐containing cell surface protrusions, whereas truncated VASP lacking the C‐terminal domain was no longer concentrated at focal adhesions. These data indicate that the C‐terminal domain is required for anchoring VASP at focal adhesion sites, whereas the central domain is suggested to mediate VASP interaction with profilin. Our results provide evidence for the structural basis by which VASP, both a target of the cAMP and cGMP signal transduction pathways and a component of the actin‐based cytoskeleton, including the cytoskeleton‐membrane interface, may be able to exchange signals between these networks.

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