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Wild-type and 16 variant maltoporins with site-directed cysteine substitutions at 14 sites were purified by a novel one-step affinity-chromatographic procedure. The trimer stability of purified proteins with C22S, C38S and G103C substitutions was reduced compared to wild-type maltoporin. Quantitative labelling with N-ethyl[14C]maleimide, cross-linking with bifunctional bismaleimides and disulphide formation was used to test the reactivity of cysteines in the folded protein. The maleimide reactivity of the residues was in the order: 152 approximately equal to 153 greater than 265 greater than 30 approximately equal to 103 approximately equal to 120 approximately equal to 154 approximately equal to 382 greater than 57 approximately equal to 146, with the other sites (22, 38, 97, 184) poorly labelled. Only cysteines at 152 or 153 permitted the formation of inter-subunit disulphide bonds suggesting these residues are located within 0.5-0.9 nm of each other in homotrimers of maltoporin. S152C and S153C as well as S154C permitted the formation of inter-subunit cross-links using bifunctional bismaleimides. The cross-linkability and the high reactivity to N-ethylmaleimide of the 150 region was consistent with the current model of the structure of maltoporin in the outer membrane; the reactivity of the other sites is also discussed within the context of this model.

Perception of a target strongly deteriorates when flanking elements are presented (crowding). Classically, crowding is explained by pooling mechanisms where target and flanker features are combined, e.g., when neurons in higher visual areas with larger receptive fields pool information from neurons in lower visual areas with smaller receptive fields. Crowding is proposed to occur in the feedforward sweep of information processing. Here, we show that crowding occurs in a highly temporal fashion, requiring substantial processing time. We presented a vertical vernier for 20ms flanked by either 2 vertical lines (one on each side of the vernier) or 2 cuboids (the cuboids contained the lines). For lines and cuboids, strong crowding occurred. When we increased the stimulus duration, crowding remained strong for the 2 lines but gradually decreased for the cuboids reaching nearly unflanked performance from a duration of about 120ms on (uncrowding). These results show that, first, uncrowding cannot be explained by simple pooling models. Second, as we proposed previously, uncrowding occurs when the flankers make up a good Gestalt (cuboids) that segregate from the target vernier. Third, the computation of good Gestalts of the cuboids takes substantial time. What matters for uncrowding is not stimulus duration per se, but processing time. When we presented only the 2 cuboids alone for 20ms, then an ISI of 120ms, and then the cuboids plus the vernier, strong uncrowding occurred. Using the Laminart model, we show how processing evolves dynamically in a recurrent fashion.