Centrosome assembly is important for mitotic spindle formation and if defec
tive may contribute to genomic instability in cancer. Here we show that in
somatic cells centrosome assembly of two proteins involved in microtubule n
ucleation, pericentrin and gamma tubulin, is inhibited in the absence of mi
crotubules. A more potent inhibitory effect on centrosome assembly of these
proteins is observed after specific disruption of the microtubule motor cy
toplasmic dynein by microinjection of dynein antibodies or by overexpressio
n of the dynamitin subunit of the dynein binding complex dynactin. Consiste
nt with these observations is the ability of pericentrin to cosediment with
taxol-stabilized microtubules in a dynein- and dynactin-dependent manner.
Centrosomes in cells with reduced levels of pericentrin and gamma tubulin h
ave a diminished capacity to nucleate microtubules. In living cells express
ing a green fluorescent protein-pericentrin fusion protein, green fluoresce
nt protein particles containing endogenous pericentrin and gamma tubulin mo
ve along microtubules at speeds of dynein and dock at centrosomes. In Xenop
us extracts where gamma tubulin assembly onto centrioles can occur without
microtubules, we find that assembly is enhanced in the presence of microtub
ules and inhibited by dynein antibodies. From these studies we conclude tha
t pericentrin and gamma tubulin are novel dynein cargoes that can be transp
orted to centrosomes on microtubules and whose assembly contributes to micr
otubule nucleation.