Sticky, Adaptable, and Many-sided: SAM protein versatility in normal and pathological hematopoietic states

Abstract

With decades of research seeking to generalize sterile alpha motif (SAM) biology, many outstanding questions remain regarding this multi-tool protein module. Recent data from structural and molecular/cell biology has begun to reveal new SAM modes of action in cell signaling cascades and biomolecular condensation. SAM-dependent mechanisms underlie blood-related (hematologic) diseases, including myelodysplastic syndromes and leukemias, prompting our focus on hematopoiesis for this review. With the increasing coverage of SAM-dependent interactomes, a hypothesis emerges that SAM interaction partners and binding affinities work to fine tune cell signaling cascades in developmental and disease contexts, including hematopoiesis and hematologic disease. This review discusses what is known and remains unknown about the standard mechanisms and neoplastic properties of SAM domains and what the future might hold for developing SAM-targeted therapies.

Keywords: cell biology; hematopoiesis; signal transduction; sterile alpha motif; transcription.

Figure 1.. Sequence and structural similarity between SAMD14/9/9L SAMs.

(A) Alignment of human SAMD14/9/9L protein sequences showing regions of identity. (B) Overlay of computationally-predicted folding of SAM domains in SAMD14/SAMD9/SAM9L. Blue - alpha helix, teal - coil, green - bend and yellow - turn. (C) Electrostatic potential maps in SAMD14/9/9L. Red, negatively charged area; blue, positive; gray, neutral.

Figure 2.. Versatility of SAM mechanisms in cellular functions.

SAMs function at distinct cellular locations within signal transduction cascades. At the plasma membrane, SAM-containing EphRs interact with Ship2 and Anks1a SAMs recruited to the receptors, opposing endocytosis and receptor degradation to prolong signal duration. T-cell function requires SASH3-SAM to promote signaling through the T-cell receptor (TCR). SAMs also promote Stem cell factor (SCF)-mediated Kit receptor signaling through Samd14-SAM, whereas Samd9l-SAM opposes signaling through the same receptor. Signal dependent activation of nuclear SAMs are essential for transcriptional regulation. ETV6 SAM polymerization enhances DNA binding, while Phc2-SAM acts as a bridge between multiple PcG repressor complex 1 (PRC1) at distal sites, both leading to transcriptional repression of target genes. Unlike these polymer forming SAMs in the nucleus, the ETS1-SAM has a docking site for ERK2 which promotes ETS1 phosphorylation and target gene expression by recruiting transcriptional co-activators like CBP/p300.

Figure 3.. SAM mechanisms in hematologic diseases.

A) Hemizygous mutation in SAM of SASH3 (Arg245Ter or Arg288Ter) leads to nonsense mediated decay of SASH3 and inhibits T cell receptor signaling causing X-linked combined immunodeficiency in humans. B) SAM-dependent polymerization of the ETV6-RUNX1 fusion proteins promote aberrant gene expression leading to cancer. Absence of the ETV6-SAM in the fusion proteins (ETV6∆SAM-RUNX1) or mutation of a critical residue(K99) within the SAM (K99R ETV6-RUNX1), prevents ETV6-RUNX1 polymerization and localization to nuclear speckles, inhibiting aberrant gene expression and leading to the rescue of cancer phenotype in cells. C) ETV6-NTRK3 fusion leads to ligand-independent dimerization and autophosphorylation of the protein and activate downstream Ras-MAPK/PI3K-Akt pathways causing constitutive gene expression leading to cancer. Mutation in the K99 of the ETV6-SAM (K99R ETV6-NTRK3) prevents SAM dependent dimerization of the ETV6-NTRK3 thus preventing constitutive activation of downstream target genes.