Mechanism of midline defect‐causing mutation P151L in MID1 revealed

Abstract

The structural complexity of a developing organism requires tight spatiotemporal control of cell-to-cell communication. This regulated intercellular messaging connects to intracellular signaling pathways that govern fundamental biological processes that facilitate organism development, such as transcription, translation, cell division, and cell migration. Common themes in elucidated molecular pathways involved in organism development include transient signals generated by opposing kinase and phosphatase activities, and by the rivaling processes of protein production and ubiquitinmediated proteasomal degradation [1]. Kinases, phosphatases, and E3 ligases are factors that play particularly critical roles in orchestrating the formation of the exquisite structural order of multicellular organisms. Additionally, new imaging technologies are revealing the importance of factors that induce mRNA localization to achieve spatially polarized protein expression in specific stages of organism development [2].The human protein MID1, which is known to play a central role in development along the ventral midline region, touches on all of these types of factors that are central to the regulation of developmental processes. MID1 is a microtubule-associated E3 ligase that targets the phosphatase PP2A for degradation [3], the microtubule association of MID1 is regulated by phosphorylation [4], active transport of MID1 along microtubules is regulated by PP2A [5], and MID1 localizes specific mRNA molecules to microtubules [6,7]. MID1 indeed is a key player in development and, as briefly described below, also in disease [6].In this issue of The FEBS Journal, Wright et al.[8] characterize the biochemical effects and elucidate the structural mechanism of a MID1 loss-of-function mutant, P151L. The P151L substitution is one of several MID1 mutations that cause midline birth defects associated with the monogenic disorder X-linked opitz G syndrome (XLOS). MID1 contains six structural domains, including a RING domain and two B-box domains (B-box1 and B-box2) that act synergistically to ubiquitinate the catalytic subunit of PP2A (PP2Ac), thereby targeting this master regulator of cellular processes to proteasomal degradation. Association of MID1 with PP2Ac requires the PP2A regulatory subunit a4, which serves as an adapter protein that binds via distinct surfaces to MID1 and to PP2Ac to form the MID1-a4-PP2Ac complex [9]. The P151L mutation is located in B-box1, the MID1 domain known to mediate interaction with a4 [10]. In this issue, Wright et al.[8] demonstrate through NMR and biochemical studies that the P151L mutant retains the overall tertiary structure of the B-box1 domain and preserves its E3 ligase activity, but loses its ability to bind to a4. This loss of interaction between B-box1 and a4 manifests biochemically as a loss of targeting and polyubiquitination of a4. The P151L mutation resides at the beginning of loop2 in B-box1, and the authors propose a structural mechanism in which the substituted Leu residue removes the conformatio...

Ubiquitin ligases: guardians of mammalian development

“…Ubiquitinated alpha4 protects PP2A from TRIM18-mediated ubiquitination and degradation. Mutant P151L TRIM18 cannot ubiquitinate alpha4 and instead ubiquitinates non-protected PP2A molecules for degradation, resulting in the pathogenic features of this syndrome such as cleft lip/palate, hypertelorism, and other midline defects [108][109][110] (Supplementary tables 1 and 2). The structural and molecular mechanisms of TRIM18-mutant associated XLOS are relatively well characterized and highlight the importance of non-proteolytic E3 ligase activity.…”