Chondroitin sulfate synthase-2/chondroitin polymerizing factor has two variants with distinct function

Area of Science:

• Biochemistry
• Molecular Biology
• Glycoscience

Background:

• Chondroitin sulfate (CS) biosynthesis involves multiple glycosyltransferases, with the chondroitin sulfate synthase-1 (CSS1)/chondroitin synthase-1 (ChSy-1) and chondroitin sulfate synthase-2 (CSS2)/chondroitin polymerizing factor complex playing a key role.
• An alternative splice variant of mouse CSS2, lacking the N-terminal transmembrane domain, was identified.

Purpose of the Study:

• To investigate the distinct roles of the original CSS2 (CSS2A) and its splice variant (CSS2B) in chondroitin sulfate biosynthesis.
• To elucidate the regulatory mechanisms governing CS chain polymerization.

Main Methods:

• Western blot analysis to confirm protein synthesis and localization (endoplasmic reticulum and Golgi apparatus) of CSS2A and CSS2B.
• Pulldown assays to study interactions between CSS2A, CSS2B, and CSS1/ChSy-1.
• In vitro glycosyltransferase assays and radiolabeling analysis in COS-7 cells to assess CS biosynthesis activity.
• Molecular modeling of CSS2A and CSS2B.

Main Results:

• Both CSS2A and CSS2B proteins are synthesized and localized in the ER and Golgi.
• CSS2A and CSS2B interact with each other and with CSS1/ChSy-1, forming multimeric complexes.
• CSS2A facilitates CS biosynthesis, while CSS2B exhibits reduced glucuronyltransferase activity and inhibits CS polymerization when co-expressed with CSS1.
• Molecular modeling supports the observed functional differences between CSS2A and CSS2B.

Conclusions:

• The study identifies distinct functional roles for CSS2 variants in regulating chondroitin sulfate chain polymerization.
• These findings provide novel insights into the complex mechanisms governing CS biosynthesis and its regulation.