  The generation of two non-identical membrane compartments via exchange of
vesicles is considered to require two types of vesicles specified by distinct
cytosolic coats that selectively recruit cargo and two membrane-bound SNARE
pairs that specify fusion and differ in their affinities for each type of
vesicles. The mammalian Golgi complex is composed of 6-8 non-identical
cisternae that undergo gradual maturation and replacement yet features only two
SNARE pairs. We present a model that explains how the distinct composition of
Golgi cisternae can be generated with two and even a single SNARE pair and one
vesicle coat. A decay of active SNARE concentration in aging cisternae provides
the seed for a cis > trans SNARE gradient that generates the predominantly
retrograde vesicle flux which further enhances the gradient. This flux in turn
yields the observed inhomogeneous steady-state distribution of Golgi enzymes,
which compete with each other and with the SNAREs for incorporation into
transport vesicles. We show analytically that the steady state SNARE
concentration decays exponentially with the cisterna number. Numerical
solutions of rate equations reproduce the experimentally observed SNARE
gradients, overlapping enzyme peaks in cis, medial and trans and the reported
change in vesicle nature across Golgi: Vesicles originating from younger
cisternae mostly contain Golgi enzymes and SNAREs enriched in these cisternae
and extensively recycle through the Endoplasmic Reticulum (ER), while the other
subpopulation of vesicles contains Golgi proteins prevalent in older cisternae
and hardly reaches the ER.
