The viral genome is not present unprotected, but is always packed in a protein coat. The protein envelope of the genome is the capsid. Capsids are composed of symmetrical building blocks, the caspsomers.
Capsomers can consist of a single polypeptide chain or they can be composed of several, different polypeptide chains (e.g. poliovirus). The complex of capsid and nucleic acid is called the nucleocapsid of a virus.
Depending on the number and composition of proteins used and the type of nucleic acid of the virus, nuekleocapsids result in 2 forms:
The capsids or nucleocapsids may represent the entire virus structure, as in rotavirus or poliovirus. This form is called "naked viruses". In contrast to this are the "enveloped viruses".
Enveloped viruses are surrounded by a double lipid envelope in which viral glycoproteins are embedded (measles, mumps, rubella virus). The cellular origin of this lipid envelope means that it can also contain cellular transmembrane proteins.
When a virus is adsorbed, an external structural element of the virus particles (capsid or envelope glycoprotein) reacts as a ligand with one or more receptors of the membrane of a cell. This binds the particles to the cell.
Examples of viral receptors are the C3d receptor (CD21) as attachment site for Epstein-Barr virus, the CD4 surface molecule as well as the chemokine receptors CCR5 and CXCR4 as receptors for HIV or the ACE receptor for COVID-19.
The choice of receptors is one of the factors that determines the tropism of the virus for different cell types.
Hof H (2019). General virology. In: Hof H, Schlüter D, Dörries R, eds Duale Reihe Medizinische Mikrobiologie. 7th, completely revised and expanded edition. Stuttgart: Thieme S 170