Nonhistone Lysine Methylation as a Protein Degradation Signal.

Protein degradation is a fundamental feature of cellular life, and malfunction of this process is implicated in human disease. Ubiquitin tagging is the best characterized mechanism of targeting a protein for degradation; however, there are a growing number of distinct mechanisms which have also been identified that carry out this essential function. For example, covalent tagging of proteins with sequestosome-1 targets them for selective autophagy. Degradation signals are not exclusively polypeptides such as ubiquitin, NEDD8, and sequestosome-1. Phosphorylation, acetylation, and methylation are small covalent additions that can also direct protein degradation. The diversity of substrate sequences and overlap with other pleotrophic functions for these smaller signaling moieties has made their characterization more challenging. However, these small signals might be responsible for orchestrating a large portion of the protein degradation activity in the cell. As such, there has been increasing interest in lysine methylation and associated lysine methyltransferases (KMTs), beyond canonical histone protein modification, in mediating protein degradation in a variety of contexts. This review focuses on the current evidence for lysine methylation as a protein degradation signal with a detailed discussion of the class of enzymes responsible for this phenomenon.

Enhanced Hyaluronan Signaling and Autophagy Dysfunction by VPS35 D620N.

The motor features of Parkinson's disease (PD) result from the loss of dopaminergic (DA) neurons in the substantia nigra with autophagy dysfunction being closely linked to this disease. A PD-causing familial mutation in VPS35 (D620N) has been reported to inhibit autophagy. In order to identify signaling pathways responsible for this autophagy defect, we performed an unbiased screen using RNA sequencing (RNA-Seq) of wild-type or VPS35 D620N-expressing retinoic acid-differentiated SH-SY5Y cells. We report that VPS35 D620N-expressing cells exhibit transcriptome changes indicative of alterations in extracellular matrix (ECM)-receptor interaction as well as PI3K-AKT signaling, a pathway known to regulate autophagy. Hyaluronan (HA) is a major component of brain ECM and signals via the ECM receptors CD44, a top RNA-Seq hit, and HA-mediated motility receptor (HMMR) to the autophagy-regulating PI3K-AKT pathway. We find that high (>950 kDa), but not low (15-40 kDa), molecular weight HA treatment inhibits autophagy. In addition, VPS35 D620N facilitated enhanced HA-AKT signaling. Transcriptomic assessment and validation of protein levels identified the differential expression of CD44 and HMMR isoforms in VPS35 D620N mutant cells. We report that knockdown of HMMR or CD44 results in upregulated autophagy in cells expressing wild-type VPS35. However, only HMMR knockdown resulted in rescue of autophagy dysfunction by VPS35 D620N indicating a potential pathogenic role for this receptor and HA signaling in Parkinson's disease.