Running Head: Nonmuscle Myosin II contributes to synaptic transmission

24 The synaptic vesicle population in a nerve terminal is traditionally divided into 25 subpopulations according to physiological criteria; the readily releasable pool (RRP), the 26 recycling pool and the reserve pool. It is recognized that the RRP subserves synaptic 27 transmission evoked by low frequency neural activity and that the recycling and reserve 28 populations are called upon to supply vesicles as neural activity increases. Here we investigated 29 the contribution of Nonmuscle Myosin II (NMMII) to synaptic transmission with emphasis on 30 the role a motor protein could play in the supply of vesicles. We used Drosophila genetics to 31 manipulate NMMII and assessed synaptic transmission at the larval neuromuscular junction. We 32 observed a positive correlation between synaptic strength at low-frequency stimulation and 33 NMMII expression: reducing NMMII reduced the evoked response, while increasing NMMII 34 increased the evoked response. Further we found that NMMII contributed to the spontaneous 35 release of vesicles differentially from evoked release suggesting differential contribution to these 36 two release mechanisms. By measuring synaptic responses under conditions of differing 37 external calcium concentration in saline we found that that NMMII is important for normal 38 synaptic transmission under high frequency stimulation. This research identifies diverse 39 functions for NMMII in synaptic transmission and suggests that this motor protein is an active 40 contributor to the physiology of synaptic vesicle recruitment. 41