BCAR1 isoforms
Isoform 1: 916 aa, calc MW= 97,7 kDa. Isoform 2: 888 aa, (alternate 5 sequence compared to variant 1) calc MW= 95,1 kDa. Isoform 3: 888 aa, (alternate 5 sequence compared to variant 1) calc MW= 95,3 kDa. Isoform 4: 888 aa, (alternate 5 sequence and alternate splice site in the substrate domain compared to variant 1 resulting in a different N-terminus and additional segment in the middle region compared to isoform 1) calc MW= 95,3 kDa. Isoform 5: 870 aa, (lacks an exon in the 5 region, alternate AUG start codon, has a different N-terminus compared to isoform 1) calc MW= 93,16 kDa. Isoform 6: 870 aa, (different N-terminus compared to isoform 1) calc MW= 93,2 kDa. Isoform 7, 868 aa (shorter, alternate 5 sequence, different N-terminus compared to isoform 1) calc MW= 93 kDa. Isoform 8, 722 aa, (alternate internal sequence compared to isoform 1, different N-terminus compared to isoform 1) calc MW= 77,6 kDa. Isoform 9: 660 aa, (shorter alternate 5 sequence, different N-terminus compared to isoform 1) calc MW= 70,7 kDa.

BCAR proteins migrate during SDS-PAGE electrophoresis at a significantly higher molecular weight than predicted from sequence analysis perhaps due to the extensive phosphorylation of BCAR proteins. Calculated MW 93,2 kDa, SDS-PAGE observed MW 130 kDa. Potential sites of human BCAR1 phosphorylation (PhosphoSitePlus): tyrosine residues aa 12, 128, 165, 192, 222, 224, 234, 249, 267, 287, 306, 327, 362, 372, 387, 410, 653, 664, 666; serine residues aa 134, 139, 292, 437, 639; and threonine residues aa 269, 326, 385. Inducers of BCAR1 phosphorylation include cell matrix adhesion, extracellular matrix rigidity, growth factors, hormones and progression through the cell cycle. Phosphorylation of BCAR1 regulates BCAR1 dependent activities through altering protein interactions, protein localization and signaling cascades (Tikhmyanova et al., 2010).

BCAR1 domains as described in Tikhmyanova et al., 2010 are shown in the schematic diagram in figure 2. The amino terminal 1-65 aa contain the Src homology 3 domain (SH3) domain that binds proline-rich PxxP ligands. The adjacent region 66-447 aa contains the substrate domain (SD) comprised of 15 YxxP motifs that when phosphorylated by tyrosine kinases provides canonical binding sites for proteins containing SH2 domains such as Crk, mechanical forces and stretching of SD may induce conformational changes that allows phosphorylation by kinases and this stretching may promote protein-protein interactions in this domain (Sawada et al., 2006). The serine rich domain within 448-610 aa (serine rich protein interaction domain) contains a four-helix bundle that functions as a scaffold for BCAR1 binding proteins such as Grb2 and 14-3-3 (Nasertorabi et al., 2004; Briknarová et al., 2005). The C-terminal domain of 746-870 aa has a potential FAT (focal adhesion targeting) domain and a helix-loop-helix domain with homology to the transcription factor Id. This region contains the YDYVHL motif that is phosphorylated during cell adhesion.

BCAR1 interacting proteins (BioGRID)
CRKII, p60-Src, PTPN12, PTK2 (FAK), RapGEF1 (C3G), NPHP1, PTPN1 (PTP1B), FES, SHIP2 (INPPL1), ARHGAP32 (p250GAP), Pyk2 (PTK2B), Fyn, CRKL, YWHAZ, SrcIN1 (SNIP), p85-alpha (PI3KR1), c-ABL (bcr/abl), Lyn, Grb2, Dock1, paxillin, TRIP6, SH-PTP2, ID2A, UHRF2, NEDD9, NCK1, VCL, SAP1, Zyxin, BCAR3 (AND-34), CD2AP, LCK, SFN, SH2D3C, JNK/SAPK1, SH3KBP1, tensin 1 (TNS1), HCK, EFS, E2F2, VPS11, HspA5, TUBA1A, GADD34, p140Cap, BCAR1 (p130CAS), PTP-PEST, CIZ, Aurora-A, 14-3-3, CHAT-H, AIP4, APC/C and CDH1.

BCAR1 is ubiquitously expressed and is reportedly detectable in all phases of the cell cycle. In lymphoid development, BCAR is expressed at higher levels in differentiated cells compared to precursors. Barretts esophagus cancer cell line compared to normal tissue 2,51 increase in BCAR1 expression (Oncomine). Colorectal cancer Ramaswamy multi-cancer there is a 4,2 fold increase in BCAR1 expression compared to other cancers (Oncomine). Gastric cancer cell line Gyorffy cell line 2 there is a 5,0 fold reduction in BCAR 1 expression (Oncomine). In lymphomas, BCAR1 expression is reduced 2,5 fold (Oncomine).

Cytoplasm, ruffles, cell junctions (Donaldson et al., 2000), nucleus (Kim et al., 2004) and focal adhesions (Nakamoto et al., 1997; Volberg et al., 1995; Winograd-Katz et al., 2009).

BCAR1 regulates numerous cellular processes such as invasion, migration, transformation, survival and drug resistance (Di Stefano et al., 2011; Brábek et al., 2004; Brábek et al., 2005) (summarized in figure 3). BCAR1 lacks intrinsic enzymatic activity, yet it is a substrate for several kinases including the Src tyrosine kinase. The original name for BCAR1 was p130CAS abbreviated from Crk-associated substrate because it was first identified as a tyrosine phosphorylated protein in cells transformed by v-src and v-crk oncogenes. BCAR1 regulates cellular behavior by controlling signaling cascades and the dynamic localization of multi-protein complexes. The BCAR1 phosphorylation state is regulated during the cell cycle. During the exit of G2, BCAR1 serine and threonine phosphorylation levels increase and these events disrupt the interactions of BCAR1 with Src and FAK and thus dissociates this complex and contributes to the disassembly of focal adhesions allowing cells to loosen matrix adhesions and thus permitting cell rounding in mitosis. The subsequent reformation of matrix adhesions promotes progression through the cell cycle from mitosis to G1 (Pugacheva et al., 2006).

There is a family containing four proteins related to BCAR1 (breast cancer resistance) that possess names related to the prior nomenclature for BCAR1 homologs in the rat and mouse. The non-human homologs of BCAR1 were named CAS for Crk-associated substrate. This family of proteins includes the protein EFS (embryonal Fyn-associated substrate) (CAS3, CASS3, EFS1, EFS2, HEFS, SIN) identified because of interactions with the Src-family kinases Fyn and Yes and maps to chromosome 14q11.2-q12. A third family member is HEF1 (human enhancer of filamentation 1 known as CASL, CAS-L, NEDD9, CAS2 and CASS2) that maps to chromosome 6p25-p24 and was isolated as a human gene that promotes filamentous growth in yeast. This screen was performed to identify regulators of the cell cycle and polarity. It was also identified as a protein that is tyrosine phosphorylated after clustering integrin β1 in T-lymphocytes. NEDD9 (neural precursor cell expressed, developmentally down-regulated 9) is a gene restricted in expression to early embryonic, but not adult mouse brain. The fourth family member is CASS4 ((HEF-EFS-P130CAS-like)/CAS4) that maps to chromosome 20q13.2-q13.31 and is the newest member of the family that was identified by genomic and transcript homology and demonstrated to function similarly to other BCAR family members. These 4 proteins are conserved from jawed vertebrates through mammals. One BCAR member is found in lower vertebrates and insects. However, no BCAR family member is detectable in C. elegans, S. cerevisiae and other lower eukaryotes.

Catalogue of somatic mutations in cancer: there are currently 10 known somatic mutations in BCAR1. Proceeding from the N-terminus to the C-Terminus of BCAR1, aa 118 proline (identified in the central nervous system), 185 alanine (identified in the central nervous system), 407 threonine (identified in breast tissue), 430 serine (upper aerodigestive tract), 583 serine (identified in prostate tissue), 592 histidine (identified in liver), 708 lysine (identified in the central nervous system), 759 threonine (identified in central nervous system), 780 valine (identified in central nervous system), 795 isoleucine (identified in upper aerodigestive tract). Mutations at aa 118 and 185 are in the substrate domain, 407 and 430 are amino-terminal to the 4 helical bundle, 583 and 592 are in the 4-helix bundle, whereas 759, 780 and 795 localize to the C-terminal domain.