LooKING at a living cell through the microscope is a bit like having a bird's eye view of Piccadilly Circus in rush hour. Vesicles and organelles show a complex two-way traffic pattern along intracellular highways of microtubules radiating out from the nucleus. Movement is either outwards (anterograde), towards the suburbs at the cell periphery, or inwards (retrograde), towards the bright lights at the cell centre. Once the black box of cell motility, organelle transport is rapidly becoming its star performer, at least as judged by the frequency of News and Views articles devoted to it. What has brought about this dramatic turri-around has been the ability to reconstitute particle movements in vitro12• The first significant success of this approach was the identification of a protein, kinesin, which has at least some of the properties expeCted of the anterognide organelle translocator3• Now, in the September issue of the Jour-nal of Cell Biology" and on page 181 of this issue\Richard Vallee and colleagues report the isolation of a protein with impressive credentials for being the retrograde organelle motor. placed on a glass microscope slide coated with MAP 1C, microtubule gliding occurs in a continuous, unidirectional fashion'. The rate of about 1.25~ tm s· 1 is comparable to that recorded for organelle motility in living cells, b1. 1t about four-to fivefold higher than previo1. 1sly reported for kinesiri l! Sing the same assay'. Most significantly, however, MAP lC and kinesin generate force in opposite directions5• The to1. 1chstone of microtub1Jle-polarity experiments are the axonemes of the green alga Chlamydomonas in which the plus and minus microtubule ends can be unambiguously distinguished (see Fig. 3 of ref. 5, on page 183). On MAP 1C-coated slides, Vallee and collaborators found axoneme gliding is always from the compact (proximal or minus) to the frayed (distal or plus) end. Kinesin, on th~ other hand, causes axonemes to glide in the opposite direction. Because in living cells microtubules are oriented with their plus ends distal to the organizing centre at the nucleus, force production by MAP lC is in the retrograde direction. Thus, MAP 1C could be the molecular motor for endocytosis, for fast retrograde transport in neurons and even for the positioning of organelles such as the Golgi apparat1. 1s and lysosotnes near