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Organelles form junctions with other organelles and the plasma membrane via specialized structures known as membrane contact sites. Membrane contact sites are regions of close apposition whereby the membranes on either side of the junction are separated by <30 nm. They are stabilized by tethering proteins that span the junction.

The ER is a particularly well‐connected organelle. It forms membrane contact sites with most organelles, including lysosomes (Figure 2.5), as well as with the plasma membrane. ER membrane contact sites are often tethered by a protein called VAP (VAP is short for the cumbersome name “vesicle‐associated membrane protein associated protein”!). VAPs are integral proteins of the ER that recognize and bind to specific target proteins on the apposing membrane (page 120). This is an example of a protein–protein interaction and we will meet many other examples throughout this book.

Membrane contact sites serve a variety of functions. They are important for the trafficking of lipids around the cell and in the transfer of calcium between compartments, for example between the ER and mitochondria.

Figure 2.5. Electron micrographs showing contact sites between the endoplasmic reticulum (ER) and lysosomes (left) and endosomes (right). Endosomes will be described in Chapter 12.

Source: Images by Bethan S. Kilpatrick, Clare E. Futter, and Sandip Patel, University College London. Reproduced by permission.