The ubiquitin-activating enzyme (E1) of the early-branching eukaryote Giardia intestinalis shows unusual proteolytic modifications and play important roles during encystation

► Giardia intestinalis E1 expression is regulated during encystation. ► The E1 enzyme is proteolytically processed and two fragments containing the N' and C' extremes are present in the cell. ► E1 is an essential gene in G. intestinalis. ► E1 localization and overexpression are related with cell differentiation. ► Our findings suggest a role for E1 in Giardia's biology.