Peters plus syndrome mutations affect the function and stability of human β1,3-glucosyltransferase

Ao Zhang,Aarya Venkat,Rahil Taujale,James L Mull,Atsuko Ito,Natarajan Kannan,Robert S Haltiwanger

doi:10.1016/j.jbc.2021.100843

Abstract

Peters Plus Syndrome (PTRPLS OMIM #261540) is a severe congenital disorder of glycosylation where patients have multiple structural anomalies, including Peters anomaly of the eye (anterior segment dysgenesis), disproportionate short stature, brachydactyly, dysmorphic facial features, developmental delay, and variable additional abnormalities. PTRPLS patients and some Peters Plus-like (PTRPLS-like) patients (who only have a subset of PTRPLS phenotypes, have mutations in the gene encoding β1,3-glucosyltransferase [B3GLCT]). B3GLCT catalyzes the transfer of glucose to O-linked fucose on thrombospondin type-1 repeats. Most B3GLCT substrate proteins belong to the ADAMTS superfamily and play critical roles in extracellular matrix. We sought to determine whether the PTRPLS or PTRPLS-like mutations abrogated B3GLCT activity. B3GLCT has two putative active sites, one in the N-terminal region and the other in the C-terminal glycosyltransferase domain. Using sequence analysis and in vitro activity assays, we demonstrated that the C-terminal domain catalyzes transfer of glucose to O-linked fucose. We also generated a homology model of B3GLCT and identified D421 as the catalytic base. PTRPLS and PTRPLS-like mutations were individually introduced into B3GLCT, and the mutated enzymes were evaluated using in vitro enzyme assays and cell-based functional assays. Our results demonstrated that PTRPLS mutations caused loss of B3GLCT enzymatic activity and/or significantly reduced protein stability. In contrast, B3GLCT with PTRPLS-like mutations retained enzymatic activity, although some showed a minor destabilizing effect. Overall, our data supports the hypothesis that loss of glucose from B3GLCT substrate proteins is responsible for the defects observed in PTRPLS patients, but not for those observed in PTRPLS-like patients.

Highlights

Developmental delay, and other systematic abnormalities at variable penetrance [1]
Peters plus syndrome (PTRPLS) mutations ablate in vitro B3GLCT enzymatic activity and the ability to rescue secretion of ADATMS20 TSR2-8 from B3GLCT−/− cells. Together these results demonstrate that the molecular mechanism for the phenotypes observed in PTRPLS patients is loss of glucose on B3GLCT substrate proteins and not an unknown function of the N-GT-like domain
These results confirm that loss of secretion of B3GLCT substrates such as ADAMTS20 from B3GLCT−/− cells is due to loss of glucose and not to the chaperone effect of an enzymatically inactive enzyme

Summary

Introduction

Developmental delay, and other systematic abnormalities at variable penetrance [1]. These patients carry intronic and/or exonic mutations in the gene encoding β1,3-glucosyltransferase (B3GLCT, formerly B3GALTL), which can be homozygous or compound heterozygous [1,2,3]. To address the questions raised above, we analyzed the domain structure of B3GLCT in more detail and demonstrated that the C-GT domain of B3GLCT is responsible for the transfer of Glc to O-Fuc on TSRs. We introduced PTRPLS/PTRPLS-like mutations individually to human B3GLCT and performed kinetic analysis of the WT and mutant forms of the enzyme using in vitro enzyme assays.

Results

Conclusion