Model of AZ organization in different brp mutants. Wild-type AZs contain BRP filaments made of both BRP isoforms in an alternating pattern (Fig. 3), which create release slots for RRP SVs. In brp^nude mutants, lacking the very C terminus, SVs tethered to the BRP cytomatrix are absent (Hallermann et al., 2010). In brp^Δ190 mutants, the number of BRP filaments is reduced (Fig. 5) because of the lack of the BRP-170 isoform. This leads to a ∼50% decrease in the number of SVs docked close to the T bar, which is also reflected by a decrease in the number of RRP vesicles (Fig. 7).