when cells migrate , they extend dynamic membrane - bound actin - rich tubular protrusions known as filopodia . formins are a family of large multi - domain proteins that nucleate and polymerize actin to form linear actin filaments like those found within filopodia . in this review formins function as dimers and nucleate actin by means of a formin homology 2 ( fh2 ) domain that binds globular actin monomers . interaction of the adjacent formin homology 1 ( fh1 ) domain with profilin effectively recruits actin monomers to the formin dimer , facilitating the polymerization process . a subset of formins , known as diaphanous - related formins ( drfs ) , bind to and drfs are rendered inactive by interaction of a c - terminal diaphanous autoregulatory domain ( dad ) with an n - terminal diaphanous inhibitory domain ( did ) ( fig .  1 ) . the binding of a rhogtpase to the n - terminal gtpase binding domain ( gbd ) contributes to disruption of this autoinhibitory interaction , which results in the activation of the drf . in the case of the drf fhod1 , phosphorylation of c - terminal serine and threonine residues by rock other domains found in drfs include a a dimerization domain ( dd ) and a coiled coil ( cc ) region , and some groups believe that the dd and did together constitute a loosely defined formin homology 3 ( fh3 ) domain . an n - terminal phospholipid - binding basic domain ( bd ) mdia1 has three stretches of basic amino acids in this domain , which allows it to localize to the plasma membrane , while mdia2 has two . in contrast , the drf daam1 has only one stretch of basic amino acids in its n - terminal , and the distribution of constitutively active daam1 is restricted to the cytoplasm . this might explain why only mdia1 and mdia2 have been implicated in the formation of membrane - bound cell surface protrusions like filopodia and lamellipodia , while mdia3lacking this bd  has only been reported to generate cytoplasmic actin structures . several rhogtpases and other proteins have been reported to bind to mdia1 - 3 . these are summarized in figure  1 . the shortest fragment(s ) known to bind the respective mdia isoforms is shown for each interacting protein . excluded from this diagram are ywk - ii , which binds a 223 aa fragment of hdia1 that shares 96% aa sequence identity with mdia1 ( 903 - 1125 aa ) , and inf2 , which binds to aa 1181 - 1262 and aa 1051 - 1193 of what might be longer splice variants of mdia1 and mdia3 respectively . mdia1 - 3 form several types of actin - based cellular structures . within the cytoplasm , mdia1 gives rise to stress fibers , and mdia2 drives the actin dynamics that power vesicle movement and creates the actin scaffold for constriction of the contractile ring during cytokinesis . at the plasma membrane , both mdia1 and mdia2 a role for formins in lamellipodial protrusion is not surprising  once thought to be structures comprised of dendritic networks of branched microfilaments , lamellipodia have recently been reported to contain linear actin filaments as well . we have found mdia3 to be capable of inducing filopodia in n1e115 neuroblastoma cells , despite not being able to detect the presence of endogenous mdia3 protein in this particular cell line . all three mdia isoforms have also been linked to invadopodia protrusion , while mdia2 alone plays a role in forming the filopodial precursors of dendritic spines . actin dynamics leading to the formation of the phagocytic cup in macrophages are believed to involve mdia1 and mdia2 too . previous studies have pointed to a role for mdia2 in mammalian filopodia . a decrease in filopodial protrusion was seen in mouse fibroblasts overexpressing constitutively active cdc42 and microinjected with anti - mdia2 antibodies , as well as cells cotransfected with activated cdc42 and a non - functional mdia2 mutant . in nih3t3 fibroblasts , constitutively active mdia2 , when overexpressed alone in b16f1 melanoma cells , also accumulated at filopodial tips . furthermore , knockdown of mdia2 protein in mouse hippocampal neurons reduced the formation of the filopodial precursors of dendritic spines . in more recent work , mdia1 - 3 have been shown to induce filopodia in neuronal cells when overexpressed on their own . however , only mdia1 was seen within filopodia , as observed by time lapse imaging of live cells . the lack of mdia2 and mdia3 in neuronal filopodia implies that these drfs might be involved in the initiation of filopodium formation but not the elongation of the structures . one possibility is that mdia2 and mdia3 generate short microfilaments that are subsequently elongated by mdia1 to form mature filopodia . this would tie in with the suggestion that mdia2 is a relatively strong nucleator but poor elongator of microfilaments , based on observations that it elongates microfilaments at a slower rate than mdia1 . the filopodia induced by full - length wildtype mdia2 appeared cylindrical and of even thickness along their length , and so did the filopodia formed by fh1fh2-mdia2 , a fragment of mdia2 that consists of only the fh1 and fh2 domains . this is unlike the club - shaped filopodia obtained by transfecting b16f1 cells with constitutively active mdia2 , where the structures are packed with shorter microfilaments at their distal ends but contain relatively few long microfilaments that extend toward the base of the protrusions . the high overexpression of constitutively active mdia2 could have resulted in endogenous mdia1 protein becoming a limiting factor on the process of filopodial microfilament elongation , giving rise to the club - shaped morphology of the filopodia . in addition , both full - length mdia2 and fh1fh2-mdia2 localized mostly to the cytoplasm  in these experiments it is likely that less of the protein was present in filopodia , and there was enough endogenous mdia1 to elongate the smaller number of short microfilaments generated , thus resulting in filopodia of even thickness along their shafts . these interpretations of the findings would further implicate mdia1 as the key mdia isoform that elongates microfilaments to form mature filopodia . it would be interesting to see if knocking out mdia1 affects mdia2-driven filopodial protrusion  will the resulting mdia2-induced filopodia be shorter in length ? mdia1 was seen throughout the shafts of filopodia when overexpressed alone or together with irsp53 or constitutively active rif in neuronal cells . this is in contrast to the  tip nucleation model of filopodium formation , where formins are expected to be found only at the tips of the protrusions . one possible explanation is that mdia1 dimers play a dual role in filopodium formation and are involved in not just polymerising the actin filaments but bundling them together as well . another possibility is that filopodia consist of short , discontinuous actin filaments that do not span the entire length of the filopodial shaft . this has been shown by cryo - electron tomography studies to be the nature of the actin filaments that constitute dictyostelium filopodia . superresolution microscopy studies would be able to reveal detail at the nanometre scale and help elucidate the specific locations of mdia1 and mdia2 , as well as other proteins associated with filopodial protrusion , within the structures with much greater accuracy . this would help in establishing a better understanding of the roles of these proteins in the various stages of filopodium formation . cdc42 works through irsp53 , which recruits to the plasma membrane the following proteins that modulate actin dynamics : n - wasp , mena , wave2 and eps8 . the rif pathway to filopodia does not require irsp53 , n - wasp , mena or wave2 . as for the mdia proteins , mdia1 appears to be the only isoform common to both pathways . in the filopodia of neuronal cells overexpressing irsp53 , mdia1 but not mdia2 was present , and was observed to interact with irsp53 within the structures . while both mdia1 and mdia2 were present in rif filopodia , only mdia1 interacted with the rhogtpase . in addition , knockdown of either of these two isoforms resulted in a decrease in rif - driven filopodium formation , while irsp53 filopodia were affected only by the silencing of mdia1 expression . taken together , it appears that mdia2 is not required for irsp53 to form filopodia , and we have found that coexpressing mdia2 with irsp53 leads to a loss of filopodia instead . it remains to be seen as to why cells need two or even moreyetundiscovered pathways to form filopodia , and why irsp53 requires only mdia1 when rif appears to require both mdia1 and mdia2 . also , rif has been shown to bind the gbd of mdia3 , however the significance of this interaction has yet to be investigated . mdia1 and possibly mdia2 are involved in stress fiber formation in addition to filopodial protrusion . these two types of actin - based cellular structures appear to be linked  in fish fibroblasts , microfilaments in filopodia can become incorporated into stress fibers , and it has been suggested that the reverse might occur in rat embryonic fibroblasts , with the actin freed up by the dissolution of stress fibers facilitating the protrusion of filopodia . rif interacts with both mdia1 and mdia2 and is able to trigger the formation of both filopodia and stress fibers . the dual role of these three proteins might point to a major role for them in controlling the balance between these two types of actin structures in cell migration . mdia2 appears to be specific for the rif - mediated pathway whereas mdia1 is required for the pathways controlled by cdc42 and rif . it remains to be seen how exactly rif utilizes two different formins , mdia1 and mdia2 , to form filopodia . how rif couples membrane deformation with actin dynamics to give rise to these structures has also yet to be resolved . apart from mdia1 and mdia2 , are there other proteins specific to the rif pathway to filopodium formation ? what potential roles do irsp53 family proteins ( irtks , mim , abba and pinkbar ) play in filopodial protrusion ? these are some of the important questions to address in future studies on mammalian filopodia .
mdia proteins are members of the formin family of actin nucleating proteins that polymerize linear actin filaments . 
 such filaments form the core of thin , tubular , membrane - bound cell surface protrusions known as filopodia , which are a major feature of mammalian cell morphology . 
 filopodia are dynamic structures that help cells sense environmental cues , and play a role in cell migration , axon guidance , angiogenesis and other processes . 
 rhogtpases bind to and control the activity of mdia proteins , and several other binding partners of the three mdia1 isoforms 
 mdia1 , mdia2 and mdia3have been documented . 
 two independent pathways controlling mammalian filopodium formation have emerged , with one driven by the rhogtpase cdc42 , and the other by rif . 
 while mdia2 has been the main formin implicated in forming filopodia , mdia1 has recently surfaced as the key formin utilized by both the cdc42 and rif pathways to drive filopodial protrusion .