Protein expression levels of the Src activating protein AFAP are developmentally regulated in brain

Preparation and Characterization of a Polyclonal Antibody against Human Actin Filament-Associated Protein-120 kD

Actin filament-associated protein-120kD (AFAP-120) is an alternatively spliced isoform of actin filament-associated protein-110kD (AFAP-110) and contains an additional neuronal insert (NINS) fragment in addition to identical domains to the AFAP-110. Unlike AFAP-110 widely expressed in tissues, AFAP-120 is specifically expressed in the nervous system and plays a role in organizing dynamic actin structures during neuronal differentiation. However, anti-AFAP-120 antibody is still commercially unavailable, and this may hinder the function research for AFAP-120. In this study, we simultaneously used the ABCpred online server and the BepiPred 1.0 server to predict B-cell epitopes in the exclusive NINS sequence of human AFAP-120 protein, and found that a 16aa-peptide sequence was the consensus epitope predicted by both tools. This peptide was chemically synthesized and used as an immunogen to develop polyclonal antibody against AFAP-120 (anti-AFAP-120). The sensitivity and specificity of anti-AFAP-120 were analyzed with immunoblotting, immunoprecipitation, and immunofluorescence assays. Our results indicated that anti-AFAP-120 could react with over-expressed and endogenous human AFAP-120 protein under denatured condition, but not with human AFAP-110 protein. Moreover, native human AFAP-120 protein could also be recognized by the anti-AFAP-120 antibody. These results suggested that the prepared anit-AFAP-120 antibody would be a useful tool for studying the biochemical and biological functions of AFAP-120.

AFAP120 regulates actin organization during neuronal differentiation

During development, dynamic changes in the actin cytoskeleton determine both cell motility and morphological differentiation. In most mature tissues, cells are generally minimally motile and have morphologies specialized to their functions. In metastatic cancer, cells generally lose their specialized morphology and become motile. Therefore, proteins that regulate the transition between the motile and morphologically differentiated states can play important roles in determining cancer outcomes. AFAP120 is a neuronal-specific protein that binds Src kinase and protein kinase C (PKC) and cross-links actin filaments. Here we report that expression and tyrosine phosphorylation of AFAP120 are developmentally regulated in the cerebellum. In cerebellar cultures, PKC activation induces Src kinase-dependent phosphorylation of AFAP120, indicating that AFAP120 may be a downstream effector of Src. In neuroblastoma cells induced to differentiate by treatment with a PKC activator, tyrosine phosphorylation of AFAP120 appears to regulate the formation of the lamellar actin structures and subsequent neurite initiation. Together, these results indicate that AFAP120 plays a role in organizing dynamic actin structures during neuronal differentiation and suggest that AFAP120 may help regulate the transition from motile precursor to morphologically differentiated neurons.

The actin cross-linking protein AFAP120 regulates axon elongation in a tyrosine phosphorylation-dependent manner

Growth cone guidance and axon elongation require the dynamic coordinated regulation of the actin cytoskeleton. As the growth cone moves, actin-dependent forces generate tension that enables protrusive activity in the periphery and drives growth cone translocation. This dynamic remodeling of the actin cytoskeleton in response to membrane tension requires activation of Src kinase. Although it has been proposed that these actin-dependent forces vary with the extent of actin cross-linking, the identity of the cross-linking protein(s) remains unknown. AFAP120 is a nervous system specific actin cross-linking protein that is regulated by Src kinase phosphorylation. Here, we report that AFAP120 is expressed and tyrosine phosphorylated in differentiating cerebellar granule cells, where it is enriched in the axon and growth cone. Over-expression of AFAP120 enhances neurite elongation in a tyrosine phosphorylation-dependent manner. These findings suggest that AFAP120 may coordinate Src signaling with the dynamic changes in the actin cytoskeleton that drive growth cone motility and axon elongation.

Analysis of the role of the leucine zipper motif in regulating the ability of AFAP-110 to alter actin filament integrity

AFAP-110 has an intrinsic ability to alter actin filament integrity as an actin filament crosslinking protein. This capability is regulated by a carboxy terminal leucine zipper (Lzip) motif. The Lzip motif facilitates self-association stabilizing the AFAP-110 multimers. Deletion of the Lzip motif (AFAP-110(Deltalzip)) reduces the stability of the AFAP-110 multimer and concomitantly increases its ability to crosslink actin filaments, in vitro, and to activate cSrc and alter actin filament integrity, in vivo. We sought to determine how the Lzip motif regulates AFAP-110 function. Substitution of the c-Fos Lzip motif in place of the AFAP-110 Lzip motif (AFAP-110(fos)) was predicted to preserve the alpha-helical structure while changing the sequence. To alter the structure of the alpha-helix, a leucine to proline mutation was generated in the AFAP-110 alpha-helical Lzip motif (AFAP-110(581P)), which largely preserved the sequence. The helix mutants, AFAP-110(Deltalzip), AFAP-110(fos), and AFAP-110(581P), demonstrated reduced multimer stability with an increased capacity to crosslink actin filaments, in vitro, relative to AFAP-110. An analysis of opposing binding sites indicated that the carboxy terminus/Lzip motif can contact sequences within the amino terminal pleckstrin homology (PH1) domain indicating an auto-inhibitory mechanism for regulating multimer stability and actin filament crosslinking. In vivo, only AFAP-110(Deltalzip) and AFAP-110(581P) were to activate cSrc and to alter cellular actin filament integrity. These data indicate that the intrinsic ability of AFAP-110 to crosslink actin filaments is dependent upon both the sequence and structure of the Lzip motif, while the ability of the Lzip motif to regulate AFAP-110-directed activation of cSrc and changes in actin filament integrity in vivo is dependent upon the structure or presence of the Lzip motif. We hypothesize that the intrinsic ability of AFAP-110 to crosslink actin filaments or activate cSrc are distinct functions.