Abstract
To describe trafficking of secretory cargo within the Golgi apparatus, the cisternal maturation model predicts that Golgi cisternae change their properties from cis to trans while cargo remains in the cisternae. Cisternal change has been demonstrated in living yeast Saccharomyces cerevisiae; however, the behavior of cargo has yet to be examined directly. In this study, we conducted simultaneous three-color and four-dimensional visualization of secretory transmembrane cargo together with early and late Golgi resident proteins. We show that cargo stays in a Golgi cisterna during maturation from cis-Golgi to trans-Golgi and further to the trans-Golgi network (TGN), which involves dynamic mixing and segregation of two zones of the earlier and later Golgi resident proteins. The location of cargo changes from the early to the late zone within the cisterna during the progression of maturation. In addition, cargo shows an interesting behavior during the maturation to the TGN. After most cargo has reached the TGN zone, a small amount of cargo frequently reappears in the earlier zone.
Highlights
In the secretory pathway, secretory cargo proteins newly synthesized in the ER are delivered to the Golgi apparatus, where they are processed and glycosylated before being sorted to their final destinations (Mellman and Warren, 2000; Emr et al, 2009)
We have succeeded in simultaneous threecolor and 4D observation to visualize a transmembrane secretory cargo together with the early and late Golgi resident proteins by the high-speed and high-resolution microscopic technology we developed, super-resolution confocal live imaging microscopy (SCLIM; Kurokawa et al, 2013)
Three-color 4D observation visualizes sequential unidirectional maturation from cis-Golgi to trans-Golgi and to the trans-Golgi network (TGN) For the analysis of behaviors of Golgi cisternae in yeast, a variety of pairs of Golgi markers residing in early and late Golgi cisternae have been used as fusions with fluorescent proteins (Losev et al, 2006; Matsuura-Tokita et al, 2006; Rivera-Molina and Novick, 2009; Suda et al, 2013)
Summary
Secretory cargo proteins newly synthesized in the ER are delivered to the Golgi apparatus, where they are processed and glycosylated before being sorted to their final destinations (Mellman and Warren, 2000; Emr et al, 2009). The mechanism of cargo trafficking within the Golgi apparatus has been a hot issue in the field of membrane trafficking (Emr et al, 2009; Glick and Nakano, 2009; Nakano and Luini, 2010; Glick and Luini, 2011). Three major models of anterograde cargo trafficking have been proposed: (a) traffic via cisternal maturation, (b) traffic by anterograde vesicular carriers, and (c) traffic via interconnected continuity of cisternae (Pfeffer, 2010; Glick and Luini, 2011). Which of these mechanisms best explains the intra-Golgi cargo trafficking still remains controversial. Recent two studies using inducible unnatural protein aggregation have led to opposite conclusions (Lavieu et al, 2013; Rizzo et al, 2013)
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