The onset of organelles by eukaryotes seemed such an innovation over the apparent pandemonium in prokaryotes lacking organizational membranes. Look closer and cells run tidy businesses whether inside or outside protein fortresses.
An endoplasmic reticulum (red) forms stable contacts with ribonucleoprotein granules (green).Though eukaryotes incorporate cellular cities for various ventures, enterprise thrives in rural cytoplasmic fields. “The cytoplasm contains an unconventional class of organelles that concentrate specific factors and resources without a limiting membrane. These membraneless organelles include ribonucleoprotein (RNP) granules. These granules are composed of nontranslating messenger RNAs (mRNAs) and associated proteins,” report American biochemist Gia Voeltz and colleagues. “Membraneless organelles are biomolecular condensates that form by coalescence of macromolecules,” explain biochemists Benoît Kornmann & Karsten Weis.
Such productive villages “also occur in the nucleus, including the nucleolus, a factory for ribosome assembly, and Cajal bodies that function in small RNA processing,” add Kornmann & Weis.
Fruitful transactions within membraneless organelles occur through low-affinity, multivalent interactions among proteins and RNA. “These weak interactions cause phase separation but do not lead to a fixed position or stoichiometry of the components within the organelle, unlike in a classical protein complex,” elaborate Kornmann & Weis.
Via genic libraries as instruction guides, sentient macromolecules, including RNP granules, cooperatively produce and process a wide variety of bioproducts.
Urban trade is established and maintained by protein complexes that tether 2 membranes together and keep organelles in proximity for hours on end. “Organelle tethering creates a platform that allows the exchange of metabolites and information,” explain Kornmann & Weis. Urbanites likewise establish exchange with rural residents at stable trading posts.
The endoplasmic reticulum (ER) has a myriad of responsibilities, including carbohydrate metabolism, lipid synthesis, glycoprotein production, and cell membrane manufacture. The ER also plays a critical role in assisting mitochondrial division and replication. The ER and mitochondria have tightly coupled dynamics via extensive contacts.
As ER maintain expertise in the status of cells via extensive information exchange, these organelles decide the pace of cellular production. Rural residents take their cue from local ER in providing timely support. “ER contact sites allow the direct exchange of macromolecules and serve as a platform for the recruitment of machineries that regulate organelle biogenesis, division, and trafficking,” notes Voeltz. ER contact sites also regulate biogenesis and fission of rural communities (RNP granules).
Cell life is an intricate enterprise of intelligent macromolecules, involving both city dwellers within membraned walls and rural cytoplasmic residents. Cells thrive only by coordinated, cooperative endeavor involving all cellular citizens.
References:
Ishi Nobu, The Science of Existence, BookBaby (2019).
Jason E. Lee et al, “Endoplasmic reticulum contact sites regulate the dynamics of membraneless organelles,” Science (31 January 2020).
Benoît Kornmann & Karsten Weis, “Liquid but not contactless,” Science (31 January 2020).