Effect of the p.His367Arg mutation on the cellular localisation of NARF

The nuclear localisation of the NARF protein has been previously described (Barton and Worman, 1999). To validate this and to analyse the impact of the p.His367Arg mutation on protein localisation, I generated expression plasmids for wild-type (further referred as NARF WT) and mutant (further referred as NARF^p.H367R) NARF, both N-terminally tagged with a Myc-tag. I transfected HeLa cells with these expression constructs to overexpress WT or mutant NARF. Subsequently, I subjected the transfected cells to immunostaining with anti-Myc antibodies, and I analysed the subcellular localisation of NARF using fluorescence microscopy. Expression of Myc-tagged NARF WT in the HeLa cells resulted exclusively in a nuclear localisation, mainly co-localising with the nuclear envelop. By contrast, Myc-tagged NARF^p.H367R was primarily present in the cytoplasm of the transfected cells, and it was no longer translocated to the nucleus as observed for NARF WT. In addition, NARF^p.H367R was not evenly distributed in the cytoplasm, but aggregated within the cytoplasm, forming puncta and clump-like structures (Figure 7a). Subsequently, I confirmed these results independently through WB analyses. I subjected the proteins extracted from the HeLa cells overexpressing either NARF WT or NARF^p.H367R to subcellular fractionation, and I isolated the cytoplasmic and nuclear fractions of the proteins and subjected them to WB analysis using anti-Myc-tag antibodies. I found that NARF WT was present in both the cytoplasmic and the nuclear fractions extracted from the HeLa cells, but I only detected NARF^p.H367R in the cytoplasmic fraction of the extracted proteins (Figure 7b); this confirmed the results obtained by immunostainings.

Figure 7: Cellular localisation of overexpressed WT NARF and mutant NARF^p.H367R proteins. (a) Immunofluorescence staining with the anti-Myc-tag antibodies (green) revealed expression of Myc-tagged WT NARF in the nucleus and expression of mutant Myc-tagged NARF^p.H367R in the cytoplasm. I used DAPI (blue) and phalloidin (red) staining to stain the DNA and to show the shapes of the cells, respectively. NARF WT was localised in the nucleus, mostly on the nuclear envelope, while mutant NARF^p.H367R was expressed in the cytoplasm and accumulated in aggregates. Scale bars = 10 µm. (b) Western blot analysis depicting the cellular distribution of the overexpressed WT NARF and mutant NARF^p.H367R proteins. The NARF WT protein was present in both fractions, but was predominantly expressed in the nuclear fraction, whereas mutant NARF^p.H367R was expressed exclusively in the cytoplasmic fraction. I used the anti-LMNA/C antibody and anti-α-tubulin as nuclear and cytoplasmic fraction controls, respectively. M = SeeBlue Plus2 Pre-Stained Protein Standard, CF = cytoplasmic fraction, NF = nuclear fraction.

Interestingly, the expression of NARF^p.H367R was always significantly lower than the expression of NARF WT. To determine whether the observed differences were attributable to inefficient transfection or the instability and degradation of the mutant protein, I transfected

DAPI NARF WT phalloidin

DAPI NARF^p.H367R phalloidin

merged

kDa

62 49

Myc-NARF CF NF

CF = cytoplasmic fraction NF = nuclear fraction

Lamin A Lamin C α-tubulin NARF WT NARF^p.H367R

M CF NF

a

b

HeLa cells with both forms of the NARF protein tagged with Myc-tag. I then extracted the cytosolic and nuclear protein fractions from 24, 48, and 72 hours after transfection and analysed them through WB using anti-Myc-tag antibodies. The WB analysis indicated that NARF WT was equally expressed over time, whereas the expression of mutant NARF dramatically decreased 48 hours after transfection. At 72 hours after transfection, NARF^p.H367R could not be detected in either the lysates (Figure 8).

Figure 8: Stability of WT NARF and mutant NARF^p.H367R. Western blot results illustrating the expression levels of overexpressed WT and mutant NARF 24, 48, and 72 hours after the transfection of HeLa cells with pCMV-Myc-N plasmids expressing either NARF WT or NARF^p.H367R. I detected overexpressed proteins using anti-Myc-tag antibodies. The NARF WT protein was expressed 72 hours after transfection, while mutant NARF^p.H367R was almost undetectable after 48 hours. M = SeeBlue Plus2 Pre-Stained Protein Standard, CF = cytoplasmic fraction, NF = nuclear fraction.

The severe effect induced by a single amino acid change in the protein sequence suggested that the conserved histidine at position 367 in NARF plays an important role for protein function. To further evaluate these findings, I designed a series of mutants, introducing a point mutation into the triplet encoding the histidine at position 367; I established six different mutant forms of NARF (named mutations 1-6, as enumerated in Table 31). Mutation 6 represented a negative control for this experiment. This represents a synonymous variant, c.1101C>T, not altering the amino acid (p.H367H).

CF NF CF NF

NARF WT NARF^p.H367R

24h 48h 72h kDa

CF = cytoplasmic fraction NF = nuclear fraction 49

Table 31: Amino acid changes in the NARF mutants. List of the mutations introduced into the NARF protein at amino acid position 367. The table reports the changes at the cDNA and protein levels and the characteristics of the newly introduced amino acids.

Name Mutation Amino Acid

Substitution

Type of Amino Acid Change Mutation 0 – patient’s mutation c.1100A>G p.H367R basic > basic

aromatic > aliphatic

Mutation 1 c. 1100 A>C p.H367P basic > neutral

aromatic > aliphatic

Mutation 2 c.1100 A>T p.H367L basic > neutral

aromatic > aliphatic

Mutation 3 c.1099 C>G p.H367D basic > acidic

aromatic > aliphatic

Mutation 4 c.1101 C>G p.H367Q basic > neutral

aromatic > aliphatic

Mutation 5 c.1099 C>T p.H367Y basic > neutral

aromatic > aromatic

Mutation 6 – negative control c.1101 C>T p.H367H –

I cloned all the mutants’ open reading frames (ORFs; 1-6) into the pCMV-Myc-N vector.

Next, I transfected HeLa cells with all the constructs and subjected them to immunostaining with anti-Myc-tag antibodies. Using fluorescence microscopy, I examined the subcellular localisation of all six of the mutants. Like NARF^p.H367R, all the additional mutants (NARF^p.H367P, NARF^p.H367L, NARF^p.H367D, NARF^p.H367Q, and NARF^p.H367Y) accumulated within aggregates in the cytoplasm of the transfected cells (Figure 9). Meanwhile, the negative control represented by a synonymous change (mutation 6, Table 1) was localised in the nucleus and associated with the nuclear envelope, as observed for NARF WT (Figure 9).

These results substantiated the important role of the histidine at position 367 in the cellular localisation of NARF. All changes at position 367 resulted in impaired nuclear import.

Figure 9: Subcellular localisation of NARF mutants carrying different amino acids at position p.H367 (NARF^p.H367P, NARF^p.H367L, NARF^p.H367D, NARF^p.H367Q, and NARF^p.H367Y). I employed immunofluorescence staining with anti-Myc-tag antibodies (red) to detect the Myc-tagged mutant proteins in the HeLa cells. All the

NARF^p.H367Y

NARF^p.H367H

DAPI NARF^p.H367P merged

DAPI NARF^p.H367L merged

DAPI NARF^p.H367D merged

DAPI NARF^p.H367Q merged

DAPI merged

NARF mutants were localised in the cytoplasm, while NARF^p.H367H, which served as the control, was localised in the nucleus. I counterstained the nuclei with DAPI (blue). Scale bars = 10 µm.

Im Dokument The role of NARF and other novel progeria-associated genes/proteins in ageing processes. (Seite 75-80)