SERCAs

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

Sarcoplasmic reticulum (SR) is a form of the smoothfaced endoplasmic reticulum (ER) in muscles. It functions as an intracellular Ca2+ store for muscle contraction. Ca2+ is energetically sequestered into the SR by Ca2+-pump/sarcoplasmic endoplasmic reticulum Ca2+-ATPase (SERCA) and released via Ca2+ release channels on stimuli (ryanodine receptor in striated muscles and inositol 1,4,5-trisphosphate receptor in most smooth muscles). Endoplasmic reticulum in non-muscle tissues also functions as an intracellular Ca2+ store. 

In striated muscles, SR is well developed to surround the myofibrils and is divided into two parts, the terminal cisternae (TC) and longitudinal tubules (LT). TC forms triad (skeletal muscle) or dyad (heart) structure with transverse tubules. The ryanodine receptor is located only in the TC, whereas the Ca2+ pump/SERCA is densely packed in both TC and LT. 

Our discussion here will concentrate on the various forms of the Ca " transport ATPases that occur in the sarcoplasmic reticulum of muscle cells of diverse fiber types and in the endoplasmic reticulum of nonmuscle cells (SERCA). The structure of these enzymes will be compared with the Ca transport ATPases of surface membranes (PMCA) [3,29-32,34] and with other ATP-dependent ion pumps that transport Na, K, andH [46,50-52]. 

The Ccr transport ATPases of sarco(endo)plasmic reticulum (SERCA)... 

The sarco(endo)plasmic reticulum Ca -ATPases of mammalian tissues can be divided structurally into three main groups (SERCA 1-3) representing the products of different genes (Table I) [8,9,11,53-57]. 

SERCA la denotes the Ca -ATPase of adult fast-twitch skeletal muscle with glycine at its C-terminus in the rabbit [53,58], and alanine at the C-terminus in the chicken [59,60]. The C-terminus of the lobster enzyme is apparently blocked [59]. 

SERCA lb is the alternatively spliced neonatal form of SERCAl, in which the glycine at the C-terminus is replaced by the alternative sequence Asp-Pro-Glu-Asp-Glu-Arg-Arg-Lys [8,9]. 

SERCA-type -ATPases from non-mammalian cells (SERCAMED) Sequences of SERCA-type Ca -ATPases were also obtained from Plasmodium yoelii [68], Anemia [69] and Drosophila [70], These enzymes are similar in size to the SERCAl- and SERCA2a-type Ca -ATPases from mammalian muscles, but based on their N- and C-terminal sequences they represent a distinct group. In spite of the wide philogenetic variations between them they all share a common N-term-inal sequence (MED) that differs from mammalian enzymes. None of the corresponding proteins were isolated and characterized. 

The molecular weights of all SERCA-type Ca " transport ATPases are in the range of 100-110 kDa. Their N-terminal sequences are similar Met-Glu-X(Ala, Asn, Glu, Asp)-X (Ala, Gly, He). The Met-Glu-X-X sequence serves as a signal for the acetylation of N-terminal methionine both in soluble and in membrane proteins [71,72]. 

There are at least five distinct isoforms of plasma membrane Ca -ATPases in mammalian tissues that differ in distribution and C-terminal sequences [34]. The molecular weight of these enzymes is in the range of 127 300-134683 (Table I) and they all contain calmodulin-binding domains [3], in contrast to the much smaller ( 110kDa) SERCA enzymes that are calmodulin independent. 

SERCA la Adult fast-twitch Rabbit, 994 109 361 MEAA EG 53... 

SERCA-type Ca -ATPases from non-mammalian cells... 

Phospholamban (PLB or PLN) is a single-pass, 52-residue integral membrane protein that regulates myocardial contractility by direct physical interaction with sarco(endo)plasmic reticulum Ca-ATPase (SERCA), a 110-kDa enzyme that maintains calcium homeostasis in the sarcoplasmic... 

The smooth endoplasmic reticulum calcium pumps (SERCA) found in brain were first identified in sarcoplasmic reticulum. The three isoforms of SERCA are products of separate genes SERCA-1 is expressed in fast-twitch skeletal muscle SERCA-2a in cardiac/slow-twitch muscle SERCA-2b, an alternatively spliced form, is expressed in smooth muscle and non-muscle tissues SERCA-3 is... 

The uniquely high resolution structural data available for the SERCAIa Ca2+ pump illuminates the structure of all P-type transporters. Unlike the Na,K pump, the catalytic subunit of the SERCA Ca2+ pumps is active and does not require association with another subunit. However, the cardiac isoform, SERCA-2a, associates with a small membrane protein, phospholamban, that can... 

The only known mechanism for Ca2+ accumulation by the endoplasmic reticulum is by means of sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pumps 381... 

The plasma membrane Ca2+-ATPase pump effects outward transport of Ca2+ against a large electrochemical gradient for Ca2+. The mechanism of the pump involves its phosphorylation by ATP and the formation of a high-energy intermediate. This basic mechanism is similar for both the plasma membrane and ER pumps however, the structures of these distinct gene products are substantially different. As discussed below, the ER pump, sometimes called a sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pump, is inhibited potently by certain natural and synthetic toxins that do not affect the plasma membrane pump. The plasma membrane pump, but not the SERCA pump, is controlled in part by Ca2+ calmodulin, allowing for rapid activation when cytoplasmic Ca2+ rises. 

FIGURE 22-3 Structures of compounds that inhibit sarcoendoplas-mic reticulum Ca2+-ATPase (SERCA) calcium pumps. 

Ca2+ entry, Ca2+-uptake into the SR by SERCA, Ca2+ extrusion from the cell and dephosphorylation of the myosin light chains. The t ype 1 phosphatase, myosin light chain phosphatase (MLCP) dephosphorylates myosin. As with MLCK its activity is physiologically regulated, e.g. its activity is decreased following phosphorylation via Rho associated kinase (Somlyo Somlyo 2000). In the uterus we have found a small but significant reduction of force, but not Ca2+ when Rho-associated kinase is inhibited (Kupittayanant et al 2001b). 

The rate of decay of the [Ca2+] following carbachol stimulation was slowed when SERCA was inhibited by CPA, by > 50%. However, the decay of the Ca2+ transient was abolished if plasmalemmal extrusion mechanisms were inhibited. 

In the uterus the SR is a pronounced and complex organelle, which appears to be under some degree of hormonal control. For example, its size, SERCA expression and release mechanisms show alteration with pregnancy. As yet there has been little study of the molecular and mechanistic processes that lead to these changes in the... 

Paul We have been talking about the SR as a site of release, as if it were one thing. There is superficial SR, deep SR and ER, with three different ryanodine receptor isoforms and at least two different SERCAs. Are these on the same vesicles, or different vesicles What s the role of the ER Do they communicate with one another There are a lot of questions about this compartmentalization and vectorial release. The mitochondria also intrigue me the SR can still function quite well after CCCP (carbonyl cyanide ///-chlorophenylhydrazone), which inhibits mitochondrial Ca2+ uptake. 

---