Sarcoplasmic/endoplasmic reticulum calcium ATPase 1
Oryctolagus cuniculus (rabbit)
Cells are some of the basic building blocks of multicellular organisms. They contain the nucleus with the genetic material and are surrounded by a lipid bilayer membrane. Within the cells many of the processes of life are carried out in an organized fashion, and cells themselves are organized internally. In muscle cells one of the substructures contained in the cell is the so-called sarcoplasmic reticulum, embedded in the bulk of the cell called the cytoplasm. The sarcoplasmic reticulum serves as a storage area for calcium. The contraction and relaxation of muscles is controlled by the flow of calcium within these cells. The flow of calcium from the cytoplasm to the sarcoplasmic reticulum causes the muscle cells to relax. The transport of calcium is controlled by a protein called the Ca2+ ATPase. This protein takes care of pumping the calcium from the cytoplasm into the reticulum. Ca2+ ATPase is a membrane protein anchored within the membrane surrounding the organelle. The part of the protein that sits within the membrane has a structure found in other membrane proteins, a collection of helices that are roughly aligned in a direction normal to the membrane plane.
Attached to the 10 transmembrane helices is the portion of the protein, called the headpiece. This headpiece, made from three distinct domains, sticks into the cytoplasm. Within the transmembrane helical part of the protein is visible a small molecule called thapsigargin. This is a tight binding inhibitor of calcium transport by Ca2+ ATPase and serves to stabilize the present structure. The presence or absence of calcium leads to large changes in the overall structure of the protein. By studying structures like the one seen here one hopes to understand in detail how such calcium pumps do their work.
Protein Data Bank (PDB)
author: Arno Paehler