Regulates asymmetric cell division (ACD), especially in stomatal-lineage cells, probably by modulating accumulation and subcellular polarization of POLAR and SPCH (PubMed:30429609, PubMed:25843888). Mediates an attenuation of MAPK signaling upon polarization of POLAR and ASK7/BIN2 in stomatal lineage ground cells (SLGCs) undergoing ACD, and relieves BIN2 inhibition of SPCH in the nucleus (PubMed:30429609, PubMed:25843888). When phosphorylated, functions as a scaffold and recruits the MAPKKK YODA, MPK3 and MPK6 to spatially reorganize the MAPK signaling pathway at the cortex of cells undergoing ACD (PubMed:25843888). Cortical polarization leads to elevated nuclear MPK6 signaling and lowered SPCH abundance in one of the two daughter cells, thus differentiating the two daughter cells after ACD (PubMed:27746029). Component of a complex made of POLAR, BASL, ASK7/BIN2 and ASK3/SK12 (PubMed:30429609). Interacts with POLAR, ASK7/BIN2 and ASK3/SK12 (PubMed:30429609). Binds to YDA when phosphorylated (PubMed:25843888). Interacts with MPK6, MPK3 and MKK5 (PubMed:25843888). Dynamic and highly polarized subcellular localization at the cortical polarity in a phosphorylation status-dependent manner (PubMed:19523675, PubMed:21903812, PubMed:30429609, PubMed:25843888, PubMed:27746029, PubMed:27422992). Polarity is only exhibited by one daughter cell after an asymmetric cell division (PubMed:27746029). ASK7/BIN2-mediated nuclear exclusion (PubMed:30429609). First localized to the nucleus in asymmetric cell division (ACD) precursors (PubMed:30429609). In asymmetrically dividing stomatal-lineage cells, before division, accumulates in a polarized crescent at the cell periphery (e.g. meristemoid mother cells (MMCs)), and, after division, localizes differentially to the nucleus (e.g. in meristemoids) and a peripheral crescent in self-renewing cells and their sisters (e.g. stomatal lineage ground cell (SLGC)) (PubMed:19523675, PubMed:30429609, PubMed:25843888, PubMed:27422992). Localized in the cell periphery at a position that minimizes the inverse distance from all points along the new division boundaries (PubMed:21903812). Mostly expressed in stomatal lineage cells including asymmetrically dividing meristemoid mother cells (MMCs) and meristemoids, and, at lower levels, in their sisters (PubMed:19523675, PubMed:27422992). Also present in vasculature (PubMed:19523675). Expressed at low levels in the epidermal pavement cells (PubMed:27422992). First observed in nuclei of epidermal cells 16 hours post germination (PubMed:19523675). Later confined to cells undergoing asymmetric divisions (e.g. stomatal lineage cells) (PubMed:19523675). Expressed in protodermal cells in young seedlings (PubMed:30429609). Copolarizes with YDA and MPK3/MPK6 in stomatal asymmetric cell division (ACD) cells (PubMed:27746029). Induced by MUTE in stomatal-lineage cells. Cortical localization of BASL requires phosphorylation mediated by MPK3 and MPK6 (PubMed:25843888). Phosphorylation promotes YDA binding (PubMed:25843888). Phosphorylation status modulates subcellular mobility (PubMed:27746029). Clustered stomatal pattern distribution in seedlings characterized by excessive numbers of small epidermal cells, due to impaired meristemoid mother cell (MMC) asymmetric division (PubMed:27746029). Abnormal non-polarized subcellular localization of POLAR and YDA in asymmetric dividing cells (PubMed:30429609, PubMed:25843888).