Lpd is an essential, evolutionary conserved regulator of the Scar/WAVE complex during cell migration in vivo.

A Xenopus class I cDNA clone, isolated from a cDNA expression library using antisera, is a member of a large family of non‐classical class I genes (class Ib) composed of at least nine subfamilies, all of which are expressed at the RNA level. The subfamilies are well conserved in their immunoglobulin‐like alpha 3 domains, but their peptide‐binding regions (PBRs) and cytoplasmic domains are very divergent. In contrast to the great allelic diversity found in the PBR of classical class I genes, the alleles of one of the Xenopus non‐classical subfamilies are extremely well conserved in all regions. Several of the invariant amino acids essential for the anchoring of peptides in the classical class I groove are not conserved in some subfamilies, but the class Ib genes are nevertheless more closely related in the PBR to classical and non‐classical genes linked to the MHC in mammals and birds than to any other described class I genes like CD1 and the neonatal rat intestinal Fc receptor. Comparison with the Xenopus MHC‐linked class Ia protein indicate that amino acids presumed to interact with beta 2‐microglobulin are identical or conservatively changed in the two major class I families. Genomic analyses of Xenopus species suggest that the classical and non‐classical families diverged from a common ancestor before the emergence of the genus Xenopus over 100 million years ago; all of the non‐classical genes appear to be linked on a chromosome distinct from the one harboring the MHC. We hypothesize that this class Ib gene family is under very different selection pressures from the classical MHC genes, and that each subfamily may have evolved for a particular function.

SummaryMIG-10/RIAM/lamellipodin (MRL) proteins link activated Ras-GTPases with actin regulatory Ena/VASP proteins to induce local changes in cytoskeletal dynamics and cell motility. MRL proteins alter monomeric (G):filamentous (F) actin ratios, but the impact of these changes had not been fully appreciated. We report here that the Drosophila MRL ortholog, pico, is required for tissue and organismal growth. Reduction in pico levels resulted in reduced cell division rates, growth retardation, increased G:F actin ratios and lethality. Conversely, pico overexpression reduced G:F actin ratios and promoted tissue overgrowth in an epidermal growth factor (EGF) receptor (EGFR)-dependent manner. Consistently, in HeLa cells, lamellipodin was required for EGF-induced proliferation. We show that pico and lamellipodin share the ability to activate serum response factor (SRF), a transcription factor that responds to reduced G:F-actin ratios via its co-factor Mal. Genetics data indicate that mal/SRF levels are important for pico-mediated tissue growth. We propose that MRL proteins link EGFR activation to mitogenic SRF signaling via changes in actin dynamics.

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