Dictyostelium, the only known non-metazoan organism to employ SH2 domain:phosphotyrosine signaling, possesses STATs (signal transducers and activators of transcription) and protein kinases with orthodox SH2 domains. Here, however, we describe a novel Dictyostelium STAT containing a remarkably divergent SH2 domain. Dd-STATb displays a 15 amino acid insertion in its SH2 domain and the conserved and essential arginine residue, which interacts with phosphotyrosine in all other known SH2 domains, is substituted by leucine. Despite these abnormalities, Dd-STATb is biologically functional. It has a subtle role in growth, so that Dd-STATb-null cells are gradually lost from the population when they are co-cultured with parental cells, and microarray analysis identified several genes that are either underexpressed or overexpressed in the Dd-STATb null strain. The best characterised of these, discoidin 1, is a marker of the growth-development transition and it is overexpressed during growth and early development of Dd-STATb null cells. Dimerisation of STAT proteins occurs by mutual SH2 domain:phosphotyrosine interactions and dimerisation triggers STAT nuclear accumulation. Despite its aberrant SH2 domain, the Dd-STATb protein sediments at the size expected for a homodimer and it is constitutively enriched in the nucleus. Moreover, these properties are retained when the predicted site of tyrosine phosphorylation is substituted by phenylalanine. These observations suggest a non-canonical mode of activation of Dd-STATb that does not rely on orthodox SH2 domain:phosphotyrosine interactions.

Dynamin is a large GTPase responsible for diverse cellular processes, such as endocytosis, division of organelles, and cytokinesis. The social amoebozoan, Dictyostelium discoideum, has five dynamin-like proteins: dymA, dymB, dlpA, dlpB, and dlpC. DymA, dlpA, or dlpB-deficient cells exhibited defects in cytokinesis. DlpA and dlpB were found to colocalize at cleavage furrows from the early phase, and dymA localized at the intercellular bridge connecting the two daughter cells, indicating that these dynamins contribute to cytokinesis at distinct dividing stages. Total internal reflection fluorescence microscopy revealed that dlpA and dlpB colocalized at individual dots at the furrow cortex. However, dlpA and dlpB did not colocalize with clathrin, suggesting that they are not involved in clathrin-mediated endocytosis. The fact that dlpA did not localize at the furrow in dlpB null cells and vice versa, as well as other several lines of evidence, suggests that hetero-oligomerization of dlpA and dlpB is required for them to bind to the furrow. The hetero-oligomers directly or indirectly associate with actin filaments, stabilizing them in the contractile rings. Interestingly, dlpA, but not dlpB, accumulated at the phagocytic cups independently of dlpB. Our results suggest that the hetero-oligomers of dlpA and dlpB contribute to cytokinesis cooperatively with dymA.

Although the distinct distribution of certain molecules along the anterior or posterior edge is essential for directed cell migration, the mechanisms to maintain asymmetric protein localization have not yet been fully elucidated. Here, we studied a mechanism for the distinct localizations of two Dictyostelium talin homologues, talin A and talin B, both of which play important roles in cell migration and adhesion. Using GFP fusion, we found that talin B, as well as its C-terminal actin-binding region, which consists of an I/LWEQ domain and a villin headpiece domain, was restricted to the leading edge of migrating cells. This is in sharp contrast to talin A and its C-terminal actin-binding domain, which co-localized with myosin II along the cell posterior cortex, as reported previously. Intriguingly, even in myosin II-null cells, talin A and its actin-binding domain displayed a specific distribution, co-localizing with stretched actin filaments. In contrast, talin B was excluded from regions rich in stretched actin filaments, although a certain amount of its actin-binding region alone was present in those areas. When cells were sucked by a micro-pipette, talin B was not detected in the retracting aspirated lobe where acto-myosin, talin A, and the actin-binding regions of talin A and talin B accumulated. Based on these results, we suggest that talin A predominantly interacts with actin filaments stretched by myosin II through its C-terminal actin-binding region, while the actin-binding region of talin B does not make such distinctions. Furthermore, talin B appears to have an additional, unidentified mechanism that excludes it from the region rich in stretched actin filaments. We propose that these actin-binding properties play important roles in the anterior and posterior enrichment of talin B and talin A, respectively, during directed cell migration.

Numerous studies have investigated the various cellular responses against genotoxic stress, including those mediated by focal adhesions. We here identified a novel type of focal adhesion remodelling that occurs under genotoxic stress conditions, which involves the replacement of active focal adhesion kinase (FAK) with FAK-related non-kinase (FRNK). FRNK stabilized focal adhesions, leading to strong cell-matrix adhesion, and FRNK-depleted cells were easily detached from extracellular matrix upon genotoxic stress. This remodelling occurred in a wide variety of cells. In vivo, the stomachs of Frnk-knockout mice were severely damaged by genotoxic stress, highlighting the protective role of FRNK against genotoxic stress. FRNK was also found to play a vital role in cancer progression, because FRNK depletion significantly inhibited cancer dissemination and progression in a mouse cancer model. Furthermore, in human cancers, FRNK was predominantly expressed in metastatic tissues and not in primary tissues. We hence conclude that this novel type of focal adhesion remodelling reinforces cell adhesion and acts against genotoxic stress, which results in the protection of normal tissues, but in turn facilitates cancer progression.

Numerous studies have investigated the various cellular responses against genotoxic stress, including those mediated by focal adhesions. Nevertheless, we here identified a novel type of focal adhesion remodelling that occurs under genotoxic stress conditions, which involves the replacement of active focal adhesion kinase (FAK) with FAK-related non-kinase (FRNK). FRNK stabilized focal adhesions, leading to strong cell-matrices adhesion, and FRNK-depleted cells were easily detached from the matrices upon genotoxic stress. This remodelling occurred in a wide variety of cells. In vivo, the stomachs of Frnk-knockout mice were severely damaged by genotoxic stress, highlighting the protective role of FRNK against genotoxic stress. FRNK was also found to play a vital role in cancer progression, because FRNK depletion significantly inhibited cancer dissemination and progression in a mouse cancer model. We hence conclude that this novel type of focal adhesion remodelling reinforces cell adhesion and acts against genotoxic stress, which results in the protection of normal tissues, but, in turn, facilitates cancer evolution.