Ribonucleoprotein particles (RNPs) are molecular complexes formed when ribonucleic acid (RNA) associates with RNA-binding proteins (RBPs) inside the cell. These structures execute and regulate the flow of genetic information, making them fundamental to life. This class of complexes includes both large, stable assemblies and smaller, transient structures, all of which dynamically manage the cell’s genetic material.
Composition and Structure
Ribonucleoprotein structures are defined by their two main components: the RNA molecule and the associated RNA-binding proteins. The RNA component often serves a dual purpose, acting as a structural scaffold for the entire complex while also providing catalytic activity for chemical reactions. For example, the RNA itself can act as a guide to recognize a specific target site, a mechanism seen in complexes that modify other RNAs.
The protein components, which can number in the dozens for larger RNPs, provide stability, regulation, and sometimes motor function. These proteins contain specific RNA-binding domains that interact with the RNA molecule through non-covalent forces, such as stacking interactions and electrostatic attractions. The complex assembly often occurs simultaneously with the RNA’s creation, meaning that proteins begin binding to the nascent RNA chain as it is still being transcribed from the DNA template. This co-transcriptional assembly requires the cell to rapidly and accurately synthesize, fold, and modify the RNA while incorporating the correct proteins.
Essential Cellular Roles
RNPs are heavily involved in controlling gene expression by managing transcripts from their creation in the nucleus to their eventual use or degradation in the cytoplasm. One major process RNPs facilitate is the modification and processing of pre-messenger RNA (pre-mRNA). The pre-mRNA is initially a long, unfinished transcript that must be edited by RNP complexes to remove non-coding segments before it can be used to make a protein.
After processing, RNPs are essential for the transport of genetic material out of the nucleus and into the cytoplasm. Messenger RNPs (mRNPs), formed when proteins bind to messenger RNA (mRNA), actively move through the nuclear pore complexes to reach their final destination.
Finally, RNPs regulate the final step of gene expression, which is translation, or protein synthesis. These complexes coordinate the entire process by regulating an mRNA’s stability, location, and conformation. By governing the rate and location of protein production, RNPs ensure that the cell has the right proteins in the right place at the right time to perform its function.
Key Types of RNPs and Their Actions
Among the most recognizable and fundamental RNPs are the ribosomes, which are the cellular factories responsible for protein synthesis. These complexes are composed of numerous ribosomal RNA (rRNA) molecules and dozens of proteins. The rRNA molecules within the ribosome are responsible for the peptidyl transferase activity, meaning the RNA itself catalyzes the formation of peptide bonds between amino acids.
Another complex RNP is the spliceosome, built from small nuclear RNPs (snRNPs), that manages the processing of pre-mRNA. The spliceosome removes non-coding introns from the pre-mRNA transcript and joins the remaining coding exons together, a process known as splicing. This precise editing requires the snRNPs to associate and dissociate in a highly ordered fashion to recognize and act upon the appropriate RNA sequences.
A third distinct RNP is telomerase, an enzyme that maintains the protective caps on the ends of linear chromosomes. Telomerase consists of a protein component with reverse transcriptase activity and an associated RNA molecule that serves as a template. This enzyme uses its RNA template to repeatedly add short, repetitive DNA sequences to the ends of chromosomes, counteracting the natural shortening that occurs during cell division.
RNPs and Human Health
The precise function and assembly of RNPs are directly linked to human health, and their dysfunction is implicated in a range of diseases. Errors in RNP assembly or stability are a feature of several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), where RNA-binding proteins like TDP-43 and FUS form pathological aggregates.
These proteins contain low-complexity domains that allow them to reversibly condense into membraneless compartments, such as stress granules, in healthy cells. However, mutations or cellular stress can lead these proteins to transition into insoluble aggregates that accumulate in the cytoplasm, disrupting cellular function and potentially causing cell death. The aggregation of TDP-43 is a neuropathological feature observed in most ALS cases and about half of FTD cases.
RNP complexes are also central to how viruses infect cells and evade the immune system. Viruses hijack host RNA-binding proteins and RNP components to promote their own replication. By manipulating or mimicking host RNP features, viruses can stabilize their own genetic material, suppress the host’s antiviral defenses, and create a cellular environment conducive to their spread.