Passage 10: N Protein
The N protein is a structurally heterogeneous, 419-amino acid-long, multidomain RNA-binding protein. Like other coronaviruses, the N protein also has
two conserved, independently folded domains, namely the N-terminal domain
(NTD) and the C-terminal domain (CTD). These two domains are connected by an
inherently disordered region (IDR) called the central linking region (LKR). The LKR
includes a Ser/Arg (SR)-rich region, which contains putative phosphorylation sites.
In addition, there are two IDRs on both sides of the NTD and CTD, which are called
N-arm and C-tail. NTD is responsible for RNA binding, CTD is responsible for RNA
binding and dimerization, and IDR is responsible for regulating the RNA-binding
activity and oligomerization of NTD and CTD.
NTD takes the shape of a right-handed fist. It consists of a four-strand antiparallel
β-fold core subdomain. The N protein plays a key role in the viral life cycle by
binding to the viral RNA genome and packing it into a helical ribonucleocapsid
(RNP) complex. This process is crucial for the assembly of the viral particle and its
subsequent release from the host cell. This process is seen in Figure 1.
Many RNA-binding proteins, especially those with a high proportion of inherently
disordered regions, participate in liquid-liquid phase separation (LLPS). The protein
LLPS is a physical and chemical phenomenon that is considered to be the key
mechanism for organizing macromolecules, such as proteins and nucleic acids, into
membrane-free organelles. These membrane cell compartments are dynamically
assembled by LLPs and endow cells with the important ability to initiate biological
functions or responses to a range of pressures. After RNA virus infection, LLPS
mediates the formation of stress granules and P-bodies. These substances play an
important role in antiviral immunity by inhibiting the translation of viral mRNA and
promoting RNA degradation. LLPS is also considered to be the key to virus assembly.
Which germ layer gave rise to the ability to combat viruses?
A) Mesoderm
B) Endoderm
C) Ectoderm
D) None of the above
Correct answer: A. First, we need to analyze the question stem to
see exactly which anatomical system or structure we are discussing. The question
stem asks about the ability to fight viruses, which indicates that it is the immune
system. There are three germ layers: Ectoderm (skin, epidermis, hair, nails, sweat
glands, nervous system, sensory organs, pituitary gland, adrenal medulla, and the
enamel of the teeth), Mesoderm (skeletal system, muscular system, cardiovascular
system, lymphatic system, excretory system, and reproductive system), and
Endoderm (lining of the gastrointestinal tract, liver, pancreas, and the lining of the
respiratory system, endocrine glands, the lining of the urinary system, and the lining
of reproductive ducts). The mesoderm gives rise to the immune system. Further, it
could be implied that the mesoderm gives rise to the immune system because it
also gives rise to the circulatory system and lymphatic system, two of the systems
essential for immune system function. Additionally, answer choice D could not be
true since every part of our body had to have originated from each of these layers
since these three layers gave rise to our body. Thus, answer choice A is the right
answer.
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