Computational modelling to integrate the effects of enzyme organisation and site-specific processing on glycosylation

Description

The non-templated polymerisation process of N-glycosylation results in a highly heterogeneous population of glycan structures. To limit randomness, the glycan processing machinery is dynamically sorted into different Golgi cisternae using vesicle trafficking that is organised by the conserved oligomeric Golgi (COG) complex. However, this organisation cannot stop stochastic enzyme competition, making it difficult to devise precise biosynthetic rules and preventing us from controlling resulting glycan distributions. We developed a computational model of glycan biosynthesis combining stochastic simulation with Bayesian fitting, to understand how altered organisation of the glycosylation machinery in the Golgi shapes different glycan distributions. We present two examples of using the model. To investigate site specific glycan processing of the conserved IgG1 Fc glycosite we first used the model to calculate alterations in organisation of the glycan processing machinery between WT and Cog4KO cells, trained on observed changes in the whole cellular N-glycan repertoire. Applying the altered glycan processing to a model IgG glycan profile produced in WT cells allowed comparison of simulated and experimental IgG glycan profiles from mutant cells. This analysis confirmed that in general glycan processing on the IgG1 Fc is slower than on a freely accessible glycosite. Surprisingly however, galactosylation was shown to be faster on the IgG than on an average cellular glycoprotein. In a second example the model was used to characterise the organisational changes of a highly selective Golgi-sorting mutant, a point mutant of Cog4 with reduced binding to the Rab30 GTPase. This mutant shows faster secretion kinetics and modelling revealed reduction of a subset of medial Golgi enzymes, implying their selective mis-sorting as a consequence of the diminished COG-Rab interaction. Predictions based on the glycan processing model thus give us mechanistic ideas for both site specific glycan processing and intra-Golgi protein sorting, which next need experimental testing.

Period	18 Apr 2024
Event title	EMBO Workshop
Event type	Conference
Degree of Recognition	International