Cell Surface Receptor Chemistry: How Structure Sets Function

Reviewed by ExaVeyra Sciences editorial team · Updated 19 October 2026

A cell surface receptor is a protein whose chemistry has been arranged to face two worlds at once. Part of it sits in the watery space outside the cell, part of it crosses the oily core of the membrane, and part of it reaches into the cytoplasm, and each part is built and decorated differently.

That arrangement decides what a receptor can bind, which partners it can reach, and what an antibody sees when a cell or a vesicle is characterized. This page sets out the shared chemistry once, so the pages on CD9, CD63, CD81 and the stromal cell marker panel can build on it.

Understanding the cell surface as a chemical interface

Roughly one quarter of human genes encode integral membrane proteins, and only part of that set reaches the plasma membrane.^1,2 A machine learning survey of the human surfaceome by Bausch-Fluck and colleagues predicted 2,886 cell surface proteins, trained on proteins experimentally confirmed at the surface.³

The same group found that the surface repertoire is cell type specific and more dynamic than the rest of the proteome.³ Their Cell Surface Protein Atlas, built by capturing surface glycoproteins from 41 human cell types, concluded that cell types differ mainly in how much of each surface protein they carry.⁴

This is the practical reason surface chemistry matters. The proteins on the outside of a cell are the ones it uses to read its surroundings, and they are also the ones a laboratory can reach with an antibody without opening the cell.³

Membrane topology decides which part of a receptor faces outward

Most surface receptors are made on ribosomes bound to the endoplasmic reticulum.¹ The signal recognition particle recognizes a hydrophobic signal sequence as it emerges from the ribosome and couples synthesis to delivery at the membrane, where the Sec61 translocon threads the chain across or into the bilayer (Fig. 1).⁵

Each stretch of about twenty hydrophobic residues can become a membrane-spanning helix, and the order in which those stretches are inserted fixes the topology (Fig. 2). Flanking positive charges tend to stay on the cytoplasmic side, a pattern known as the positive-inside rule.⁶

• Type I single-pass proteins place the amino terminus outside the cell and cross the membrane once. CD44 and CD105 are built this way.⁶
• Type II single-pass proteins keep the amino terminus inside and display the carboxyl end outside.⁶
• Multipass proteins cross the membrane several times. The tetraspanins CD9, CD63 and CD81 cross it four times.^7,8
• GPI-anchored proteins have no membrane-spanning segment and are held by a glycolipid. CD73 and CD90 are anchored this way.^9,10

Not every protein uses the same insertion route. Guna and colleagues showed that the ER membrane protein complex acts as an insertase for tail-anchored proteins with moderately hydrophobic segments,¹¹ one of several dedicated pathways reviewed by Hegde and Keenan.¹

Glycosylation gives each receptor its outer chemistry

Glycosylation is the most abundant and diverse modification of proteins in eukaryotic cells.¹² N-linked glycans are attached to asparagine within an asparagine, any residue, serine or threonine sequence as the protein enters the endoplasmic reticulum, and they are then trimmed and rebuilt as the protein moves through the Golgi.¹²

Those early glycans double as a folding checkpoint. Trimming of glucose residues engages the lectin chaperones calnexin and calreticulin, and only correctly folded proteins are released to continue toward the surface.¹³

The finished sugars change what a receptor looks like to the outside world (Fig. 3). They add mass, shield parts of the protein surface, and can form or mask antibody epitopes, which is why heavily glycosylated proteins such as CD63 run far above their predicted size on a Western blot.¹²

Lipid anchors place receptors in particular membrane neighborhoods

At least 150 human proteins are attached to the membrane by a glycosylphosphatidylinositol anchor added to their carboxyl terminus.¹⁴ The anchor holds them in the outer leaflet of the bilayer only, and it associates them with ordered membrane domains often described as rafts.¹⁴

S-acylation works from the other side. It adds a long-chain fatty acid, usually palmitate, to cytoplasmic cysteines through a thioester bond, and it is the only fully reversible lipid modification of proteins.¹⁵ Tetraspanins are heavily palmitoylated, which helps them assemble with partner proteins.⁷

Rafts themselves are now understood as packed domains driven by the preferred association of cholesterol with saturated lipids.¹⁶ A receptor with the right anchor gathers into these domains with selected partners, which is one way the membrane concentrates signaling components.¹⁶

Receptors work in organized assemblies

Few surface proteins act as isolated units. Integrins pair into heterodimers and gather into adhesion complexes that sense the stiffness and composition of the surrounding matrix, transmitting signals in both directions across the membrane.¹⁷

Tetraspanins take organization further. They bind one another and partner proteins such as integrins and immunoglobulin superfamily members, assembling tetraspanin-enriched microdomains that position receptors next to the molecules they work with.⁷

The CD system gives each surface molecule a shared name

CD stands for cluster of differentiation. The Human Leukocyte Differentiation Antigens workshops grouped monoclonal antibodies that recognized the same molecule, and over ten workshops more than 400 molecules received formal CD designations.¹⁸

The nomenclature is approved by the International Union of Immunological Societies and sanctioned by the World Health Organization.¹⁸ A CD number therefore names both a protein and the antibodies that detect it, which is why the same numbers appear on research papers and characterization reports alike.

Surface chemistry is how cells and vesicles are identified

Mesenchymal stromal cells are a clear example. The 2006 International Society for Cellular Therapy criteria require expression of CD105, CD73 and CD90 (Fig. 4) and the absence of hematopoietic markers, in addition to plastic adherence and differentiation in culture.¹⁹

Proteomic surveys show how much more is present. Niehage and colleagues identified 888 proteins after biotin capture of the bone marrow stromal cell surface, including about 200 plasma membrane proteins and 41 CD markers,²⁰ and Lv and colleagues reviewed how candidate markers vary with tissue source.²¹

Extracellular vesicles inherit membrane proteins from the cells that release them. The tetraspanins in particular are so consistently present that CD9, CD63 and CD81 have become the conventional vesicle markers,⁷ which is the subject of the next pages in this series.

Where to go from here

The tetraspanin family page explains the four-pass fold that CD9, CD63 and CD81 share, and each of those three proteins has its own reference. The quality references explain how a marker panel is measured and reported on a certificate of analysis.

Licensed practices that want characterization data for a specific lot can request the certificate of analysis once their account is verified. Our team can also walk through what each marker on a panel measures.