# Cellular protein maturation and degradation

> **NIH NIH R01** · UNIVERSITY OF MASSACHUSETTS AMHERST · 2020 · $339,392

## Abstract

Summary
The vast majority of the ~7,000 proteins that traffic through the mammalian secretory are modified by one or
more glycans. These alterations include the modifications of Asn (N-linked) and Ser/Thr (O-linked) residues.
Carbohydrates appended to proteins can assist with protein folding, quality control and trafficking, or control
their activity and function. In this proposal, we will study the mechanism and role of the addition of the
hexose epimers of glucose and mannose in the endoplasmic reticulum (ER).
 Protein maturation is monitored by a quality control process that evaluates the structural integrity of
maturing nascent chains, and permits the passage of properly folded proteins. Alternatively, non-native
proteins are marked for ER retention so that the defect can be repaired, or if irreparable, targeted for
degradation. Calnexin and calreticulin are ER carbohydrate binding molecular chaperones that direct the
folding and trafficking of secretory pathway cargo by selectively binding to monoglucosylated side chains on
maturing proteins. Therefore, to a large extent the glucosylation state of a glycoprotein controls its flow
proteins in the secretory pathway. The glucosylation state is controlled by the UGGT family members
UGGT1 and UGGT2 that appear to selectively modify immature or non-native clients to support persistent
chaperone binding. These soluble UGGTs transfer glucose from UDP-glucose to maturing cargo in the early
secretory pathway. In the first two aims of this proposal, we will test the hypothesis that the UGGTs are
central quality control gatekeepers that control the flux of proteins through the ER by examining their
specificity and the role of reglucosylation in the cell.
 The hexose epimer mannose is also added to proteins in the ER. In contrast to reglucosylation,
mannosylation involves the transfer of mannose by a membrane embedded transferase from a dolichol-P-
precursor directly to Ser/Thr residues to form an O-glycosidic bond. There are two families of putative O-
mannosyltransferases that reside in the ER membrane in mammalian cells, the POMTs (POMT1 and 2) and
the recently discovered TMTCs (TMTC1-4). Little is known about their mechanisms of action and the
function of adding a mannose to maturing proteins in the ER. We have recently found that TMTC3 is
involved in the O-mannosylation of E-cadherin and that E-cadherin’s O-mannosylation aids in cell adhesion
and neurodevelopment. Mutations in TMTC3 are associated with the neurodevelopment diseases,
underscoring the biological significance of this post-translational modification. Specific aim 3 is to
understand the mechanism and significance of O-mannosylation in the ER. The long-term goal of this
project is to understand the process of hexose addition including the substrate selection and modification
steps, and how these post-translational modifications control the trafficking and functions of proteins.

## Key facts

- **NIH application ID:** 10052659
- **Project number:** 2R01GM086874-21
- **Recipient organization:** UNIVERSITY OF MASSACHUSETTS AMHERST
- **Principal Investigator:** Daniel N. Hebert
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $339,392
- **Award type:** 2
- **Project period:** 1999-04-01 → 2024-07-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10052659

## Citation

> US National Institutes of Health, RePORTER application 10052659, Cellular protein maturation and degradation (2R01GM086874-21). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10052659. Licensed CC0.

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