Abstract
SSO genes have been isolated as multicopy suppressors of sec1-1, a
mutation in a late-acting SEC gene causing accumulation of post-Golgi vesicles
at the restrictive temperature, and shown to encode the so-called target
membrane SNAP receptors (t-SNAREs) in the plasma membrane of the yeast
Saccharomyces cerevisiae.The Sso proteins are type II membrane proteins that
are capable of posttranslational membrane insertion independent of signal
recognition particles (SRPs).Thus, they might be inserted to the plasma
membrane directly instead of entering the endoplasmic reticulum (ER) and being
transported to their site of action along the biosynthetic pathway.The
initial membrane insertion site of Sso2p in the secretory pathway was studied
by transient expression of Sso2p in animal cells, which are superior to yeast
for such morphological studies.Sso2p was shown to be first inserted into the
ER and then transported to the plasma membrane via the secretory pathway.The
Sso proteins are likely to be rate-limiting factors in the secretory machinery
of S. cerevisiae because their overproduction enhanced by several fold the
secretion of both a yeast protein, invertase, and a heterologous reporter
protein, Bacillus a-amylase.Secretion to the periplasm was enhanced in the
Sso2p-overproducing strain, consistent with a previously suggested role for
plasma membrane t-SNAREs as anchors in targeting/fusion of the secretory
vesicles to the plasma membrane.The secretion enhancement by overproduction of
components of the secretory machinery offers a novel, more generally
applicable approach to increase the rather modest secretory capacity of S.
cerevisiae.The secretory pathway of yeast has been considered to contain other
bottle necks e.g. in the ER and the Golgi complex.Interestingly,
overexpression of a newly discovered gene, SEB1, shown to encode a novel
subunit of an ER translocon, also resulted in enhanced secretion.SEB1 was
cloned as a multicopy suppressor of sec15-1, a late-acting mutant gene.SEC15
shows an extensive pattern of genetic interactions with other late-acting SEC
genes and is, thus, considered to have a central function in post-Golgi
transport.It encodes a component of a multisubunit complex called the exocyst,
which is implicated in targeting/fusion of the secretory vesicles to the
plasma membrane as an effector of Sec4p.Surprisingly, the nonessential SEB1
gene encodes the evolutionarily conserved b subunit of the Sec61p complex,
which functions in ER translocation.This unexpected finding prompted further
studies on genetic interactions between SEB1 and the genes encoding exocyst
subunits.Interestingly, SEB1, unlike its close homologue, SEB2, could suppress
mutations in all of the exocyst genes.Furthermore, overproduction of the
other two components of the ER translocon, Sec61p or Sss1p, could also
suppress defects in many of the exocyst mutants.The double disruption
seb1Dseb2D in combination with either of two mutant genes encoding exocyst
components, sec10 or sec15, caused synthetic lethality, further strengthening
the evidence for genetic interactions.The genetic interactions observed were
rather specific for the two complexes, the ER translocon and the exocyst.In
addition, overexpression of SEC1 or SSO2 could also suppress all the exocyst
mutants except one and on the other hand all the genes encoding components of
the Sec61p complex could suppress sec1-1 when overexpressed.These results
suggest a closer interplay between the ER translocon, exocyst complex and
plasma membrane t-SNARE than has previously been anticipated.
Original language | English |
---|---|
Qualification | Doctor Degree |
Awarding Institution |
|
Supervisors/Advisors |
|
Award date | 11 Jun 1999 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-5381-0 |
Electronic ISBNs | 951-38-5382-9 |
Publication status | Published - 1999 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- yeasts
- Saccharomyces cerevisiae
- genetic interactions
- enhanced secretion
- ER translocation
- exocytosis
- exocyst
- heterologous proteins
- SS02
- SEB1
- SEB2
- SNAP receptors
- plasma membrane