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yeast, Sec62p, Sec63p, Sec71p and Sec72p form a tetrameric protein com-
plex that may function as a receptor in this type of translocation process.
Several chaperones are known to participate, including Sec61 ( Wilkinson
et al., 1997 ).
Another important part of protein synthesis is folding and proper assem-
bly of the nascent polypeptide. To accomplish this, the ER contains a variety
of proteins that assist in the process. Protein disulfide isomerase (PDI) and
ERp57, for example, are thio-oxidoreductases that catalyze disulfide bond
formation by means of the oxidative capacity provided by ER oxidoreduc-
tion 1 (ERO1). Glucose-regulated protein 78 (BiP or GRP78), a chaperone
of the heat-shock protein family, recognizes and stabilizes unfolded pro-
teins, and participates in posttranslational translocation ( Ni and Lee, 2007 ).
Another set of proteins is responsible for folding quality control, a process
that will be described later in the text.
2.2.2. Endoplasmic Reticulum and Calcium Homeostasis
Calcium is recognized as one of the most important second messengers
in the cell; it participates in a wide variety of cellular processes, including
protein synthesis, muscle contraction, gene expression, secretion, cell cycle,
metabolism and apoptosis ( Coe and Michalak, 2009 ). Given its diverse func-
tions, intracellular free calcium concentrations are tightly regulated. In this
respect, a series of buffer proteins, pumps and carriers of calcium partici-
pate in this process, serving to diminish or increase calcium concentrations
according to cell requirements.
The ER is the principal organelle involved in calcium homeostasis, and
in turn, many of its diverse functions, like protein folding and glycosylation,
are calcium-dependent by virtue of the fact that the enzymes involved are
calcium-dependent ( Kuznetsov et al., 1992 ). Cytoplasmic calcium regula-
tion depends on the activity of a series of transporters in the ER membrane.
Sarcoplasmic/endoplasmic reticulum calcium-ATPase (SERCA) is a cal-
cium pump that imports calcium from the cytoplasm into the ER lumen,
thereby maintaining low cytoplasmic calcium levels ( Strehler and Treiman,
2004 ). On the other hand, the RyR and the inositol 1,4,5-trisphosphate
receptor (IP3R) are calcium channels that release this ion back into the
cytoplasm ( Marks, 2001 ; Zalk et al., 2007 ; Taylor and Tovey, 2010 ).
2.2.2.1. Sarcoplasmic/Endoplasmic Reticulum Calcium-ATPase
SERCA is a type P ATPase pump that transports two calcium ions in
exchange of the hydrolysis of one ATP molecule, functioning against a
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