Fungi, like all other forms of life, possess ribosomes, which are the molecular machines responsible for manufacturing proteins within the cell. These structures read the genetic instructions carried by messenger RNA (mRNA) and assemble amino acids into functional proteins. Fungi are classified as eukaryotic organisms, and their ribosomes are larger and more complex than those found in bacteria.
The Universal Role of Ribosomes
Ribosomes are often described as the protein factories of the cell because they execute translation. This process involves taking the coded message from mRNA and translating it into a specific sequence of amino acids. The resulting chain folds into a functional protein required for nearly every cellular activity.
This machinery is functionally identical across all living things, ensuring genetic information is consistently converted into the physical building blocks of life. Protein synthesis is highly energy-intensive, and a cell’s ability to grow and respond to its environment is tied to the efficiency of its ribosomes. Without these structures, a cell cannot create the enzymes, structural components, or signaling molecules necessary for survival.
Fungal Ribosomes: Structure and Location
The ribosomes found in the cytoplasm of fungal cells are classified as 80S ribosomes, a designation based on their sedimentation rate (Svedberg units, S). The 80S structure is composed of two distinct parts: a smaller 40S subunit and a larger 60S subunit. The 40S subunit contains one ribosomal RNA (rRNA) molecule, the 18S rRNA, along with approximately 33 proteins.
The 60S subunit is significantly larger, containing three different rRNA molecules (5S, 5.8S, and 28S rRNAs) and about 49 associated proteins. This larger, protein-rich 80S structure is a defining characteristic of eukaryotic cells, contrasting with the smaller 70S ribosomes found in prokaryotes. Within the fungal cell, these 80S ribosomes are either freely suspended in the cytoplasm or bound to the endoplasmic reticulum, where they synthesize proteins destined for secretion or membrane insertion.
The structural difference between the 80S and 70S ribosome is partly due to additional stretches of ribosomal RNA, known as expansion segments, present only in the eukaryotic version. Fungi also possess a second class of ribosomes within their mitochondria, the organelles responsible for energy production. These mitochondrial ribosomes (mitoribosomes) are smaller 70S-type structures, reflecting the ancient evolutionary origin of mitochondria as symbiotic bacteria.
Why Ribosome Differences Matter for Medicine
The structural variations between different types of ribosomes have implications for medical treatments, particularly in the development of antifungal drugs. The primary challenge is the close architectural similarity between the fungal 80S cytoplasmic ribosome and the human 80S cytoplasmic ribosome. This similarity makes it difficult to design drugs that target fungal protein synthesis without causing severe side effects by interfering with human cells.
Despite this hurdle, the subtle structural distinctions present in the fungal 80S ribosome are being explored as targets for new medications. Researchers are focusing on unique elements, such as specific rRNA expansion segments or the geometry of the ribosomal E-site, which differ slightly from the human version. Successfully targeting these small differences allows for selective toxicity—the principle of killing the pathogen without harming the host.
This approach is becoming important because many existing antifungal drugs target the fungal cell wall or membrane, and resistance to these agents is rising, particularly for pathogens like Candida albicans. Halting protein synthesis is a powerful mechanism against pathogens, demonstrated by the success of antibacterial antibiotics that exploit the difference between bacterial 70S and human 80S ribosomes. Disrupting the rapid production of ribosomes, necessary for the swift growth and virulence of pathogenic fungi like Aspergillus fumigatus, represents a promising new strategy against difficult-to-treat infections.