Formins

Formins have been found in all eukaryotes studied.[1] In humans, 15 different formin proteins are present that have been classified in 7 subgroups.[6] By contrast, yeasts contain only 2-3 formins.[7]

Formins are characterized by the presence of three formin homology (FH) domains (FH1, FH2 and FH3), although members of the formin family do not necessarily contain all three domains.[8][9] In addition, other domains are usually present, such as PDZ, DAD, WH2, or FHA domains.

The proline-rich FH1 domain mediates interactions with a variety of proteins, including the actin-binding protein profilin, SH3 (Src homology 3) domain proteins,[10] and WW domain proteins. The actin nucleation-promoting activity of S. cerevisiae formins has been localized to the FH2 domain.[4] The FH2 domain is required for the self-association of formin proteins through the ability of FH2 domains to directly bind each other, and may also act to inhibit actin polymerization.[11][12] The FH3 domain is less well conserved and is required for directing formins to the correct intracellular location, such the mitotic spindle, or the projection tip during conjugation.[13][14] In addition, some formins can contain a GTPase-binding domain (GBD) required for binding to Rho small GTPases, and a C-terminal conserved Dia-autoregulatory domain (DAD). The GBD is a bifunctional autoinhibitory domain that interacts with and is regulated by activated Rho family members. Mammalian Drf3 contains a CRIB-like motif within its GBD for binding to Cdc42, which is required for Cdc42 to activate and guide Drf3 towards the cell cortex where it remodels the actin skeleton.[15] The DAD binds the N-terminal GBD; this link is broken when GTP-bound Rho binds to the GBD and activates the protein. The addition of the DAD to mammalian cells induces actin filament formation, stabilizes microtubules, and activates SRF mediated transcription.[15] Another commonly found domain is an armadillo repeat region (ARR) located in the FH3 domain.

The FH2 domain, has been shown by X-ray crystallography to have an elongated, crescent shape containing three helical subdomains.[16][17]

Formins also directly bind to microtubules via their FH2 domain. This interaction is important in promoting the capture and stabilization of a subset of microtubules oriented towards the leading edge of migrating cells. Formins also promote the capture of microtubules by the kinetochore during mitosis and for aligning microtubules along actin filaments.[18][19]

• MBInfo - Formin mediated actin nucleation