| Comments |
P.312 middle column 2nd paragraph: "Global cytoplasmic concentrations of cytokinesis proteins (Table 1) range from 0.04 μM formin Cdc12p (600 molecules per cell) to 63.2 μM actin (1.43 million molecules per cell). The concentration of myosin-II light chain Cdc4p (4.8 μM) is 10-fold higher than that of Myo2p (0.45 μM), which is in keeping with evidence for functions beyond its partnership with myosins (refs 11–14). Most of the proteins studied are more abundant in S. pombe than in haploid strains of S. cerevisiae (table S2), in part because of the 1.7-fold higher protein content per cell of S. pombe (refs 15, 16)." P.312 right column bottom paragraph: "The biochemical reactions depend on local concentrations, so [investigators] used fluorescence and [their] standard curve (Fig. 1C) to measure local concentrations. Actin patches concentrate at cell poles and at the cell-division site where the cell wall grows (Fig. 2B). The molecular composition of each patch varies over its 20-s lifetime (ref 24) as it forms, matures, and dissociates (refs 24, 25), so measurements on patches with random ages vary considerably (Fig. 2B and Table 1 standard deviations 38 to 62% of means). Assuming that the ratio of YFP-actin to native actin is at least as high in patches as the cell as a whole and assuming that patches are spheres 250 nm in diameter (ref 26), an average patch has at least 2700 actin molecules (minimal local concentration 530 μM). Patches also contain 202 Arp2/3 complexes (40 μM with 1:1:1:1:1 stoichiometry of 5 subunits), 90 capping proteins Acp2p (18 μM), and a remarkable number, 507, of the actin filament crosslinker fimbrin Fim1p (100 μM). The fractions of each protein in patches range from 6 to 21% (Table 1)." |