Drosophila vision is mediated by inputs from three types of photoreceptor neurons: R1–R6 mediate achromatic motion detection while R7 and R8 constitute two chromatic channels. Neural circuits for processing chromatic information are not known. Here we identified the first-order interneurons downstream of the chromatic channels. Serial-EM revealed that small-field projection neurons Tm5 and Tm9 receive direct synaptic input from R7 and R8, respectively, and indirect input from R1–R6, qualifying them to function as color-opponent neurons. Wide-field Dm8 amacrine neurons receive input from 13–16 UV-sensing R7s and provide output to projection neurons. Using a combinatorial expression system to manipulate activity in different neuron subtypes, we determined that Dm8 neurons are both necessary and sufficient for phototaxis to ultraviolet in preference to green light. We propose that Dm8 sacrifices spatial resolution for sensitivity by relaying signals from multiple R7s to projection neurons, which then provide output to higher visual centers.

SUMMARY How CNS neurons form appropriately sized dendritic fields to encounter their presynaptic partners is poorly understood. The Drosophila medulla is organized in layers and columns, and innervated by medulla neurons dendrites and photoreceptor axons. Here we show that three types of medulla projection (Tm) neurons extend their dendrites in stereotyped directions and to distinct layers within a single column for processing retinotopic information. In contrast, the Dm8 amacrine neurons form a wide dendritic field to receive ~16 R7 photoreceptor inputs. R7- and R8-derived Activin/TGF-β selectively restricts the dendritic fields of their respective postsynaptic partners, Dm8 and Tm20, to the size appropriate for their functions. Canonical Activin signaling promotes dendritic termination without affecting dendritic routing direction or layer. Tm20 neurons lacking Activin signaling expanded their dendritic fields and aberrantly synapsed with neighboring photoreceptors. We suggest that afferent-derived Activin regulates the dendritic field size of their postsynaptic partners to ensure appropriate synaptic partnership.

Human (h) DNA topoisomerase I has been identified as a major SUMO1 target in camptothecin-treated cells. In response to TOP1-mediated DNA damage induced by camptothecin, multiple SUMO1 molecules are conjugated to the N-terminal domain of a single TOP1 molecule. To investigate the molecular mechanism of SUMO1 conjugation to TOP1, an in vitro system using purified SAE1/2, Ubc9, SUMO1, and TOP1 peptides was developed. Consistent with results from in vivo studies, multiple SUMO1 molecules were found to be conjugated to the N-terminal domain of a single TOP1 molecule. Systematic analysis has identified a single major SUMO1 conjugation site located between amino acid residues 110 and 125 that contains a single lysine residue at 117 (Lys-117). Using a short peptide spanning this region, we showed that a poly-SUMO1 chain was assembled in this peptide at Lys-117. Interestingly, a Ubc9-poly-SUMO1 intermediate had accumulated to a high level when the sumoylation assay was performed in the absence of hTOP1 substrate, suggesting a possibility that the poly-SUMO1 chain is formed on Ubc9 first and then transferred en bloc onto hTOP1. This is the first definitive demonstration of the assembly of a poly-SUMO1 chain on protein substrate. These results offer new insight into hTOP1 polysumoylation in response to TOP1-mediated DNA damage and may have general implications in protein polysumoylation.Human SUMO1 (small ubiquitin-related modifier) (also named UBL1, PIC1, GMP1, SMT3C, and sentrin in the literature) is a ubiquitin-like protein (1-3). It shares about 18% identity to ubiquitin (4). Human SUMO1 and its yeast homolog, Smt3p, also share a similar activation and conjugation pathway with ubiquitin but employ distinct sets of E1 2 and E2 enzymes (5-9). The E1 enzymes for activating human SUMO1 and yeast Smt3p are the heterodimeric proteins SAE1/SAE2 and Aos1p/Uba2p, respectively (6, 9). Ubc9 is the only E2 identified for SUMO1/Smt3p, although a dozen of E2 enzymes have been identified for ubiquitin in yeast (10 -14). Recently, proteases that specifically activate SUMO1/Smt3p precursors and cleave SUMO1/Smt3p from their protein conjugates have been identified in yeast and mammalian cells (15)(16)(17)(18). Many proteins such as RanGAP1 (19), promyelocytic leukemia protein (20), IB␣ (21), RAD51, RAD52 (22, 23), p53 (24), and centromere proteins (25, 26), which have diverse functions, have been shown to interact with Ubc9/SUMO1 or be covalently modified by SUMO1. SUMO1 appears to primarily target nuclear proteins (23). Although the function of SUMO1 is still elusive, there are a few interesting observations that may shed light on the function of SUMO1. First, Pods (promyelocytic leukemia protein oncogenic domains, also called nuclear bodies or ND10) (27) and the nuclear envelope appear to be the major site of localization of SUMO1 conjugates in the nucleus (28). Second, SUMO1 and ubiquitin appear to share the same conjugation site(s) on some target proteins (e.g. IB␣ and MDM2) (21). Third, stress-caused protein unfolding (e.g. heat s...