Directed cell migration occurs many times during embryogenesis. For example, in many species, primordial germ cells (PGCs) migrate after specification to the site of the future gonad. This migration involves PGC polarisation and PGC responsiveness to external guidance cues. In zebrafish, the chemokine Cxcl12a regulates directed migration, whereas the Rho GTPase Rac1 regulates polarisation. But what controls Rac activity? Fang Lin and co-workers now report that signalling mediated by the G protein subunits Gβ and Gγ (Gβγ) regulates Rac activity in zebrafish PGCs (see p. 57). The researchers show that PGCs defective for Gβγ signalling, like those with reduced Rac activity, fail to polarise and fail to migrate actively in response to directional cues. They also show that PGCs require Gβγ signalling for polarised activation of Rac and for maintenance of their overall Rac levels. These and other results suggest that, during PGC migration in zebrafish, Gβγ signalling regulates Rac activity to control the cell polarity that is needed for PGC responsiveness to chemokine signalling.

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