Section on Biological Chemistry

Lawrence A. Tabak, D.D.S., Ph.D., Chief

Mission

The Section on Biological Chemistry conducts basic research on biosynthesis, structure and function of glycoproteins, placing a special emphasis on mucin-type O-glycans.

Overview

Our laboratory studies the functions and biosynthesis of O-glycans. Mucin- glycoproteins are heavily decorated with carbohydrate side-chains, termed O-glycans, which are often clustered within repeating amino acids sequences of the protein (tandem repeats).

Functionally, membrane-bound mucins are involved in signal transduction events whereas secreted mucins contribute to the formation of extracellular matrix or to the gel-like mucus coat which envelopes mucosal surfaces of the body thereby forming the most exterior face of the innate immune system. Although it is known that O-glycans are ubiquitous among proteins, the precise nature of the “O-glycome” remains to be defined. We have approached this by both top-down and bottom-up proteomic studies as well as investigations of the substrate specificities of the multi-gene family of enzymes that are responsible for the formation of O-glycans, the UDP-GalNAc:polypeptide N-Acetylgalactosaminyltransferases (ppGalNAcTs).  Although no consensus amino acid sequence has emerged that is both necessary and sufficient for O-glycan formation, two classes of ppGalNAcTs – those which readily glycosylate peptide directly (“naked” or unglycosylated peptide) and those which will only act on glycopeptide substrates (peptides previously decorated with O-glycans) have been identified. This strongly implies that there is a hierarchical acquisition of O-glycans. Why Nature requires such precision in selecting which specific amino acids will be glycosylated is not yet known, although it may have to do with the ability of surface carbohydrates to form a “polyvalent array”, enabling cells or molecules to sample each other before entering into specific, long-term relationships via high affinity interactions with either carbohydrates and/or proteins.

Current Projects:

Our major efforts are focused on the roles played by ppGalNAcTs during development and furthering our understanding of the mechanisms that underlie ppGalNAcT function. Specific projects include:

1. Understanding the role(s) played by ppGalNAcTs in early development using sea urchins (S. purpuratus) as a model [Tom Beres; in collaboration with Lynne and Bob Angerer, NIDCR, NIH].
2. Deconvoluting the role(s) played by ppGalNAcTs in heart development using mice as a model [Yu Guan, in collaboration with several colleagues at NINDS and NHBLI, NIH].
3. Dissecting the role(s) played by ppGalNAcTs in development of mouse testes (Tom Beres; Ruei-Shiuan Lin, and Yu Guan, in collaboration with Jessie Mager, U. Mass.]
4. Analyzing the role(s) played by ppGalNAcTs in lipid metabolism [Tom Beres; Ruei-Shiuan Lin in collaboration with A.G. Holleboom, Jan Albert Kuivenhoven, and colleagues, University of Amsterdam].
5. Determining the “rules” underlying ppGalNAcT substrate specificity [Tom Beres; Ruei-Shiuan Lin in collaboration with Tom Gerken, Case Western University].
6. Evaluating potential inhibitors of ppGalNAcTs [Tom Beres;Yu Guan;Ruei-Shiuan Lin in collaboration with Suzanne Walker, Harvard  University].
7. Identification of the molecular mechanisms key to the transport of mucin-type O-glycans through the biosynthetic and endocytic pathways; Elucidation of the protein machinery that regulates ppGalNAcTs transport and activity through the biosynthetic pathway [Raul Rojas]

Tabak Group Members

Please click on individual lab members’ names to review their ongoing efforts in greater detail:

Tom Beres, Ph.D.
Raul Rojas, Ph.D.
Yu Guan, Ph.D.
Ruei-Shiuan Lin, Ph.D.