There isn't one single membrane protein responsible for connecting cells to each other. Instead, several different types of membrane proteins contribute to cell-cell adhesion, each with its own mechanism and function. Here are some examples:

1. Cell Adhesion Molecules (CAMs):

* Cadherins: These are calcium-dependent transmembrane proteins that mediate cell-cell adhesion in a variety of tissues. They bind to similar cadherins on adjacent cells, forming strong connections. Examples include E-cadherin (epithelial cells), N-cadherin (neural cells), and P-cadherin (placental cells).

* Integrins: These are heterodimeric transmembrane proteins that bind to extracellular matrix (ECM) components and can also interact with other cells. They play a crucial role in cell adhesion, migration, and signaling.

* Immunoglobulin superfamily (IgSF) CAMs: These are a diverse group of transmembrane proteins that share structural similarities with antibodies. They are involved in various functions, including cell adhesion, immune responses, and neuronal development.

2. Gap Junction Proteins:

* Connexins: These are transmembrane proteins that form channels connecting the cytoplasm of adjacent cells. These channels allow for the passage of small molecules and ions, facilitating communication and coordination between cells.

3. Tight Junction Proteins:

* Occludins and Claudins: These are transmembrane proteins that form tight junctions, which seal the space between cells, preventing the passage of fluids and solutes. They play a crucial role in maintaining tissue integrity and regulating permeability.

Therefore, the specific membrane protein responsible for connecting cells to each other depends on the type of cell junction involved and the function it needs to perform.