Is arrestin translocation driven by diffusion or molecular motors?
Most of the recent discussion on the mechanisms of protein translocation in rods and cones was centered on whether it is driven by diffusion (cf. Peet et al., 2004; Nair et al., 2005) or by the action of molecular motors carrying arrestin and transducin along axonemal microtubules (cf. McGinnis et al., 2002; Peterson et al., 2005). As we calculated previously for transducin translocation (Strissel et al., 2004), both the observed rate and the volume of protein translocation are much more compatible with diffusion. Data obtained in this study reinforce this argument. With the onset of illumination, up to 5% of the total arrestin translocates from the inner to the outer segments each minute (Fig. 3). This is equivalent to the rate of ∼3, 000, 000 molecules per minute, which is nearly 1000-fold faster than the reported rate of rhodopsin delivery into rod outer segments [∼4300 molecules per minute (Liu et al., 1999)], viewed as a benchmark for a rapid motor-driven protein transport process
