PRMT5 C-terminal Phosphorylation Modulates a 14-3-3/PDZ Interaction Switch

Abstract

Edited by John M. DenuPRMT5 is the primary enzyme responsible for the deposition of the symmetric dimethylarginine in mammalian cells. In an effort to understand how PRMT5 is regulated, we identified a threonine phosphorylation site within a C-terminal tail motif, which is targeted by the Akt/serum-and glucocorticoid-inducible kinases. While investigating the function of this posttranslational modification, we serendipitously discovered that its free C-terminal tail binds PDZ domains (when unphosphorylated) and 14-3-3 proteins (when phosphorylated). In essence, a phosphorylation event within the last few residues of the C-terminal tail generates a posttranslational modification-dependent PDZ/ 14-3-3 interaction "switch." The C-terminal motif of PRMT5 is required for plasma membrane association, and loss of this switching capacity is not compatible with life. This signaling phenomenon was recently reported for the HPV E6 oncoprotein but has not yet been observed for mammalian proteins. To investigate the prevalence of PDZ/14-3-3 switching in signal transduction, we built a protein domain microarray that harbors PDZ domains and 14-3-3 proteins. We have used this microarray to interrogate the C-terminal tails of a small group of candidate proteins and identified ERBB4, PGHS2, and IRK1 (as well as E6 and PRMT5) as conforming to this signaling mode, suggesting that PDZ/14-3-3 switching may be a broad biological paradigm.Arginine methylation is a common PTM 3 that alters roughly 0.5% of all arginine residues in the cells. There are three types of arginine methylation: monomethylarginine, asymmetric dimethylarginine, and symmetric dimethylarginine (1). PRMT5 is one of nine PRMTs, and it is responsible for the vast majority (Ͼ95%) of the symmetric dimethylarginine modifications (2). PRMT5 was first characterized as a transcriptional repressor for cyclin E1 (3), and in this context, it methylates histone H3R8me2s, H2AR3me2s, and H4R3me2s (4). An epigenetic silencing role for PRMT5 has also recently been reported for the cell cycle inhibitor p21 (5).However, PRMT5 clearly has a number of non-histone substrates that are localized to the cytoplasm and the plasma membrane (6). In the cytoplasm, PRMT5 forms part of the methylosome and methylates a number of splicing factors (7). In keeping with these observations, the conditional deletion of PRMT5 in neural stem cells leads to defects in the core splicing machinery, reduced constitutive splicing, and massive alterations in alternative splicing profiles (8). Thus, this arginine methyltransferase has key biological roles that are associated with each of the major cellular compartments (the nucleus, the cytoplasm, and the plasma membrane), although little is known about how the activity and localization of PRMT5 in these different compartments are regulated.There is an emerging interest in establishing how signal transduction pathways communicate with chromatin and regulate changes to the epigenetic landscapes (9). It is likely that enzymes like PRMT5 may be marked by different ...

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“…More recently, PRMT5 was described to be phosphorylated in its C‐terminus by the Akt and glucocorticoid‐inducible kinases on T634. This event triggers a 14–3‐3/PDZ interaction switch important for the targeting of PRMT5 to the plasma membrane …”

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